1. 1 Composition, properties and classification of substances (new curriculum standard)
Preparation strategy
According to the characteristics and laws of college entrance examination in recent years, we should pay attention to several aspects when reviewing this question.
1. Pay special attention to the understanding of "material quality", one of the seven basic physical quantities in the international system of units. We should not only know that it is an important physical quantity to establish macro-and micro-relations, but also understand its connotation and extension, and build a knowledge network system centered on "material quality". The more important question is about Avon Gadereau constant, which involves many concepts. In addition, the calculation of the amount of matter and the application of the amount of matter in the calculation of chemical equations.
2. Understanding of the concept of redox reaction. Looking at the development trend of college entrance examination in recent years, redox reaction not only pays attention to examining basic concepts, but also tends to combine the balance of redox reaction equation with the analysis and inference of substances, especially to design new problem background and thinking ladder from daily application.
3. Ion reaction: There are three types of questions in this kind of topic: one is to examine ionic equation's writing. Judging from the expression of college entrance examination questions, in addition to investigating the writing of typical ion reaction equations involved in middle school chemistry textbooks, more and more attention is paid to the application of knowledge transfer, that is, the examination of information given questions. The second is to judge whether the ion inverse program is right or wrong. 3. The ion * * * exists on the condition that there is no chemical reaction between the ions in the solution, such as metathesis reaction, redox reaction, mutually promoted hydrolysis reaction and complexation reaction. In the process of analysis and judgment, we should not only be familiar with the ion reaction when common ions cannot exist, but also pay attention to the requirements and limitations of the topic and the interaction between various ions.
4. Energy is a hot issue in social production and life. When reviewing, we should understand the concepts of combustion heat, porcelain, heat and reaction heat, make clear the writing of thermochemical equations, strengthen the connection and expansion with electrochemistry and chemical balance, firmly grasp the essence of Goose's law and skillfully use it.
Section 1 Composition, properties and classification of substances
Click on the exam outline
1. Understand the concepts of mixture and purity, simple substance and compound, metal and nonmetal.
2. Understand the concepts of acid, alkali, salt and oxide and their relationships.
3. Understand that colloid is a common dispersion system.
4. Understand the differences and connections between physical changes and chemical changes.
5. Understand the schematic diagram of atomic structure, molecular formula, structural formula and simplified representation of structure.
6. Be familiar with the valence of common elements. Can correctly write the chemical formula (molecular formula) according to the valence, or judge the valence according to the chemical formula.
Core point breakthrough
First, the classification of substances and the relationship between various substances
1. Composition and classification of substances
Dial:
(1) Criteria for judging whether it is pure or not:
A. It has fixed components, such as Fe3O4. Things made up of molecules of the same kind are pure.
Error-prone point:
Substances composed of the same element are not necessarily pure, such as diamond and graphite mixed into a mixture;
Substances with the same atom are not necessarily pure, such as 16O2 and 16O3 mixed into a mixture.
B.it has a fixed melting point. If glass has no fixed melting point, it is a mixture.
C. crystalline hydrate is pure.
(2) mixture: polymers are all mixtures; Dispersions are all mixtures.
Common inorganic mixtures should be recorded in the main series:
Ⅰ A Alkaline lime (NaOH+CaO) plant ash (main component K2CO3)
Ⅱ A marble (mainly CaCO3), fluorite (mainly CaF2), calcium carbide (mainly CaC2) and scales (CaCO3+Mg (OH) 2).
Iia thermite (aluminum powder and some metal oxides)
Iva glass, cement, ceramics, sodium silicate (water glass, that is, aqueous solution of sodium silicate), black powder.
ⅴ a calcium superphosphate (CaSO4+Ca(H2PO4)2, namely calcium superphosphate), aqua regia (volume ratio (concentrated hydrochloric acid: concentrated nitric acid) = 3: 1).
VII A bleaching powder
ⅷ pig iron
Six gases: coal-water gas (CO+H2), natural gas (mainly methane), blast furnace gas (CO, CO2 and N2), coke oven gas (H2+CH4), pyrolysis gas (ethylene, propylene and butadiene) and liquefied petroleum gas (propane and butane).
Organic matter: formalin, grease, coal, petroleum, coal tar, kerosene, gasoline, vaseline, etc.
(3) Hydrides: nonmetallic hydrides, that is, gaseous hydrides (including water) that are often extracted, are all molecular crystals; Common metal hydrides are sodium hydride and calcium hydride, both of which are ionic crystals.
(4) Comparison of four identical relations
2. Mutual transformation of substances
Dial:
As can be seen from the diagram of mutual transformation of substances
(1) mutual transformation of various substances
① From the vertical view, we can see the transformation relationship from simple substance to salt.
② From the horizontal view, we can see the changing relationship between metal and nonmetal, alkali and acid.
(2) the main chemical properties of various substances
For example, it can be seen that the main chemical properties of acids are that acids can react with basic oxides and bases to form salts and who can react with salts to form new acids and salts; It can react with active metals to produce salt and hydrogen. Therefore, when exploring the properties of strange substances, we can first analyze the categories of substances and then predict which substances it may react with.
Canon training 1(20 10? Shandong college entrance examination? T9？ 4 points) and oxygen are two nuclides, indicating Avon Garderot constant. The following statement is correct.
A. Structures that are isomorphic to each other
B. method different from extranuclear electron configuration
C. the mutual transformation between and can be achieved through chemical changes.
D. Under standard conditions, 1. 12L and 1. 12L both contain 0. 1 oxygen atoms.
The proposition is intended to examine the concept of "four similarities" and a deep understanding of the nature of chemical changes and the molar volume of gases.
Atoms are the smallest unit of chemical change, and the change between atoms does not belong to chemical change; Atomic recombination produces other substances, which is a chemical change. The molar volume of gas is related to any different gas molecules, regardless of the size of the gas particles themselves.
Select d as the standard solution.
Items A, 2 and 2 belong to the same simple substance, so A is wrong: Item B, the number of electrons of two oxygen atoms is equal, and the electron configurations outside the nucleus are also equal, so B is wrong; The transition between 16O and 18O is a change between nuclei, belonging to the same substance, not a chemical change, so c is wrong; Under the standard conditions of 1. 1.2l, the amount of O2 is 0.05mol and the number of oxygen atoms contained is 0. 1NA, so d is correct.
Category expansion
1. allotrope: Different elements formed by the same element are called allotropes. Such as O2 (oxygen) and O3 (ozone), red phosphorus (phosphorus) and white phosphorus (P4), diamond and graphite.
(1) allotropes have different physical properties and similar chemical properties. ② The reason for the different nature is the different structure.
(2) Aliens can be transformed into each other, which is a chemical change, not a redox reaction. For example:
2. Different nuclides of the same element are called isotopes, and the transformation between nuclides belongs to physical changes.
3. Alien bodies are mixed together and belong to a mixture rather than a pure substance, such as metal stone and graphite. Although they are all composed of carbon, they have different structures, so they are mixed together to form a mixture.
4.H2, D2, T2 and DT are pure when mixed together.
5. The method to distinguish the concept of "tetramer" should be clear about the research object, such as atoms whose isotopes are the same element, allotropes are simple substances of the same element, isomers refer to molecules with the same molecular formula but different structures, and isomers mainly refer to the same organic matter with similar structure (same category) and one or several -—CH2 differences.
Second, the classification of oxides and their relationship
Dial:
1. Relationship between acid oxides and anhydrides: all acid oxides are anhydrides, for example, SO2 is the anhydride of H2SO3; Anhydrides are not always acidic oxides. For example, acetic anhydride is not an acidic oxide. Multiple acids can correspond to the same anhydride. For example, the anhydrides of phosphoric acid and metaphosphoric acid are P2O5, and the anhydrides of silicic acid and orthosilicic acid are SiO2.
2. The corresponding relationships between acidic oxides and basic oxides, and between nonmetallic oxides and metal oxides are as follows:
(1) Acid oxides are not all nonmetallic oxides, such as Mn2O7 nonmetallic oxides are not always acid oxides, such as CO, NO, NO2, etc. As for NO2, it disproportionates with alkali to produce salt and water, which are only nonmetallic oxides, with 2NO2+2NaOH = NaNO2+NaNO3+H2O.
(2) Basic oxides are all metal oxides; Metal oxides are not necessarily basic oxides, such as Mn2O7, Al2O3, etc.
3. Oxides can also be divided into: ordinary oxides, peroxides, superoxides and odorous oxides. For example: K2O, K2O2, KO2, KO3.
4. It is wrong that acidic oxides must not react with acids, for example, SiO2+4hf = SiF4 = ↑+2H2O.
Canon Training 2(20 10? Shanghai roll? 5T) The following judgment is correct.
A. the anhydride must be oxide b, and there must be chemical bonds in the crystal.
C. the basic oxide must be metal oxide D. The bond angle in a regular tetrahedron molecule must be109 28'
The purpose of the proposition is to check the classification, crystal structure and valence bonds in the molecules of oxides.
In chemistry learning, some chemical concepts are easily confused, for example, oxides and oxygenated compounds are not exactly the same: oxides are composed of two elements, one of which is oxygen, while oxygenated compounds can only be composed of oxygen, such as acetic anhydride, which is an oxygenated compound rather than an oxide.
Standardized answer: C This question examines the classification of substances, the structure of crystals, and the configuration of molecules. Most of the anhydrides are oxides, but acetic anhydride (C4H6O3) is not an oxide, and A is wrong; Inert gases are all monoatomic molecules, and there is only intermolecular force in their crystals, and there is no chemical bond, which is B fault; In regular tetrahedron, the bond angle of white phosphorus molecule is 60o, and d is wrong.
Probe into the real questions of college entrance examination
1.(20 10? Shandong? 13T) The following inference is correct.
A silicon dioxide is an acidic oxide, which can react with sodium hydroxide solution.
B.Na2O and Na2O2 have the same constituent elements and are also the same as CO2 reaction products.
C.Co, NO and NO2 are all air pollution gases, which can exist stably in the air.
D. The newly made chlorine water is acidic, and a small amount of purple litmus test solution is dripped into it, and the solution turns red after full oscillation [source: Subject Network].
The proposition intends to investigate the special properties of oxides, the classification and properties of substances and the related properties of substance inspection.
Standard answer Answer: A. Acid oxides can react with alkali to form oxides of salt and water, so A is correct, because it is generated by reaction and it is generated by reaction, so B is wrong; There will be a reaction in the air, so c is wrong; D is wrong because the newly-made chlorine water contains a small amount of purple litmus, which first turns red and then fades.
2.(20 10? Jiangsu? Water is one of the most precious resources. The following statement is correct.
A. the electronic form of A.H2O is
B At 4℃, the pH=7 of pure water is 7.
In c, the sum of mass is twice the sum of protons.
D.273K, 10 1kPa, the average distance between water molecules: (gaseous) > (liquid) > (solid).
Proposition intention This topic mainly examines the basic chemical terms about water.
The idea should be familiar with the molecular formula, molecular structure, electronic and chemical terms of common substances. [Source: Learning+Discipline+Network Z+X+X+K]
Normative answer: In item c. A, water is a valence compound, and the electronic type of its molecule is; In item B, the ionization degree of water increases with the increase of temperature. Item C, in a molecule, the mass number is 20 and the proton number is 10. Item d, when the temperature and pressure are constant, can only be in one state. To sum up, we can see that item C is chosen in this question.
3.(2009? The following statement is correct.
① The formation of estuary delta is usually related to the properties of colloid.
(2) A large number of steel materials are used in the reconstruction of Sichuan disaster area, and steel materials are alloys.
③ Barium sulfate used in barium meal is a weak electrolyte.
④ Silicon in solar panels is at the boundary between metal and nonmetal in the periodic table of elements.
⑤ Commonly used tap water disinfectants are chlorine and chlorine dioxide, both of which contain polar bonds.
⑥ In the amphibious bus, the rubber material used for sealing is polymer compound.
A.①②③④ B.①②④⑥ C.①②⑤⑥ D.③④⑤⑥
This proposition aims at investigating the classification of matter.
Select b as the standard solution. According to the nature of colloid ① correct; Steel is an alloy of iron and carbon, which is correct; Barium meal is barium sulfate, which is a strong electrolyte. Although barium sulfate is insoluble, the water-soluble part is indeed completely ionized, which is wrong. Silicon is located in the main Ⅳ A group of the third period of the periodic table of elements, at the junction of metals and nonmetals. 4 correct; Chlorine is a simple substance composed of nonpolar bonds, ⑤ wrong; Rubber is a polymer compound, ⑥ correct.
Theme simulation exercise
First, multiple choice questions
1. The following statement is true ()
A. A large number of fossil fuels are used, which is conducive to "energy saving and emission reduction"
B. Sugar, oil and protein are all high molecular compounds.
C. vitamin c is reducible and plays an antioxidant role in the human body.
D. Adding water purification agents such as aluminum salt and iron salt to seawater can desalinate seawater.
2.(20 1 1? Analogy is one of the commonly used methods to study the properties of substances, which can predict the properties of many substances. But analogy is relative and cannot go against objective facts. The following analogy speculates that the statement is correct ().
① It is known that Fe and S can directly combine to form F eS, and it is speculated that Cu and S can directly combine to form CuS.
② It is known that CaCO3 reacts with dilute nitric acid to generate CO2, and it is speculated that CaSO3 reacts with dilute nitric acid to generate SO2.
③ It is known that CO2 molecule is a linear structure, and it is inferred that CS2 molecule is also a linear structure; ④ It is known that Fe can react with CuSO4 solution, and it is speculated that Fe can also react with AgNO3 solution.
⑤ NaOH is called strong electrolyte, and its saturated solution has strong conductivity, and Ca(OH)2 is also a strong electrolyte, so it is speculated that its saturated solution has strong conductivity.
A.①③④b .①②⑤c .③④d .③④⑤
3. Production and life are inseparable from various chemicals. The following statement is incorrect.
A kind of "carbon nano-foam" is called the fifth form of elemental carbon, which is an allotrope of graphite.
B: The spacesuit worn by Shenzhou VII astronauts is mainly composed of silicon carbide, ceramics and fibers. It is a new inorganic nonmetallic material.
C. protein, oil and plastics are all organic polymer compounds.
D, (,as raw materials, as products) meet the requirements of "green chemistry"
Lesson preparation resources
1.(2009? The following chemical terms are used correctly.
A. schematic diagram of atomic structure of sulfur atom:
Electronicity of B.B.NH4Cl;
C. oxygen atoms with 10 neutrons in the nucleus:
D. structural formula of p-chlorotoluene:
Select c as the standard solution. Item A is the schematic diagram of the ion structure of sulfur ion, item B should be written as Cl-, and item D stands for o-chlorotoluene, so items A, B and D are wrong; Since the proton number of O is 8, oxygen has 10 neutrons.
The atom should be expressed as, so item C is correct.
2.(2009? Fujian college entrance examination) can realize the direct transformation between the following elements is ()
A. silicon-sulfur-copper-iron
B. Because the oxides of silicon, copper and iron do not react with H2O to generate corresponding acids and bases, A, C and D do not conform to the transformation relationship in the question.
3.(2009? National Volume II) The following statement about solution and colloid is correct ()
A. The solution is electrically neutral and the colloid is charged.
B. When electrified, the solute particles in the solution move to two poles respectively, and the dispersoid particles in the colloid move to one pole.
C. The movement of solute particles in solution is regular, while the movement of dispersoid particles in colloid is irregular, that is, Brownian motion.
D. When a beam of light passes through the solution and colloid respectively, the latter will appear obvious light band, while the former does not.
The colloid of item D is still electrically neutral, but the colloidal particles are charged. The dispersive particles of item b move to the poles respectively; The movement of solute particles in item C solution is irregular, so A, B and C are wrong.
4.(2009? Li Ji, Guangdong) The incorrect use of the following chemical terms is ().
A. the structural schematic diagram of na+is
B. The chemical formula of soda ash is Na2CO3.
C the structural formula of polyethylene is CH2 CH2.
The valence of chlorine in perchloric acid (HClO4) is +7.
C. polyethylene is a polymer with the structural formula, so item C is wrong.
5.(2008? The development of chemical synthesis technology in the twentieth century has made great contributions to the improvement of human health and quality of life. The following groups of substances are obtained by chemical synthesis.
A. glass cellulose penicillin
B. urea salt polyethylene
C. polyester washing powder aspirin
D. timely rubber indium phosphide
To solve this problem, we should pay attention to the following two points:
(1) Naturally occurring substances do not need to be obtained by chemical synthesis.
(2) Substances that can be obtained through physical changes do not need chemical synthesis.
Choose C for your answer. The cellulose in A is rich in nature, such as cotton; The salt in B is rich in seawater and does not need artificial synthesis; In D, it also exists in nature in large quantities, such as crystals; All the substances in C are synthesized.
The chemical changes easily overlooked by the conventional method (1) are the transformation between different simple substances formed by the same element (for example, O3 is transformed into O2), the electrolyte solution is conductive, and the color change of NO2 gas when heated or cooled.
(2) There must be the breaking and formation of chemical bonds in chemical changes, but the changes accompanying the breaking of chemical bonds are not necessarily chemical changes, such as the dissolution of HCl in water and the ionization of molten sodium chloride.
(3) Chemical changes are often accompanied by luminescence, exotherm and discoloration, but the changes accompanied by luminescence, exotherm or discoloration are not necessarily chemical changes. For example, when oxygen is cooled to-184℃ at atmospheric pressure, O2 becomes a light blue liquid.
(4) Although the fission and fusion of atoms form new substances, they do not belong to the study of chemical changes in middle schools.
Not only chemical changes have taken place, but also physical changes have taken place. For example, when a candle is lit, the melting of paraffin is a physical change, while the burning of paraffin produces CO2 and H2O is a chemical change.
6.(2008? A cooperative learning group discussed and analyzed the following statements: ① crude salt and acid rain are mixtures; ② Biogas and water gas are both renewable energy sources; ③ Ice and dry ice are pure substances and compounds; ④ Stainless steel and coins in circulation are all alloys; ⑤ Hydrochloric acid and vinegar are both compounds and acids; 6. Soda soda and hydrated lime are both alkali; ⑦ Soymilk and fog are colloids. The above statement is correct ()
A.①②③④ B.①②⑤⑥
C.③⑤⑥⑦ D.①③④⑦
D. This question examines the classification of substances. ① Both crude salt and acid rain contain various components, belonging to a mixture; ② Biogas belongs to renewable energy and water gas belongs to non-renewable energy; ③ The chemical components of ice and dry ice are H2O and CO2 respectively, which belong to pure compounds; (4) Stainless steel is a kind of steel made by adding elements such as chromium and nickel on the basis of ordinary steel. At present, coins in circulation have different face values and materials, but they are all alloy; ⑤ Hydrochloric acid and vinegar are mixtures rather than compounds; 6 soda ash, namely sodium carbonate, is salt, not alkali; ⑦ Soymilk belongs to liquid sol and fog belongs to aerosol, both of which are colloids.
Teaching reflection:
20 1 1 version of the second round of special review study plan for senior high school chemistry: 1.2 Common measurement of chemistry (new curriculum standard)
Click on the exam outline
1. Understand the unit of substance quantity-mole, molar mass, molar volume of gas and the meaning of Avogadro constant.
2. Calculate according to the correlation between the amount of substance and the number of particles (atoms, molecules, ions, etc.). ) and gas volume (under standard conditions).
3. Understand the method of preparing solution with a certain amount and concentration of substances.
Core point breakthrough
1. Study on Avogadro Constant
The application of Avon Gadereau constant is essentially the conversion between physical quantities centered on the quantity of matter, so it is necessary to pay special attention to accurately grasp the connotation of related concepts. Mainly:?
1. state problems, such as SO3 is solid and pentane is liquid under standard conditions. The molar volume of gas (22.4 L/mol) is suitable for gases under standard conditions, and Avon Gadereau's law is suitable for gases under the same conditions. ?
2. The molar mass of special substances, such as D2O 18o 2, etc. ?
3. The number of atoms in a substance molecule, such as O3, white phosphorus, rare gas, etc.
4. Chemical bonds in certain substances, such as silicon dioxide, P4, carbon dioxide, etc.
5. Some ions or atomic groups in substances are hydrolyzed, such as Al3+ in Na2CO3 and AlCl3. ?
6. Common reversible reactions, such as incomplete ionization of 2NO2 N2O4 and weak electrolyte. ?
7. When 22.4 L/mol is applied, whether the substance is gas or not, and whether the gas is in a standard state. ?
8. The number of electron transfer in some reactions, such as?
Special reminder:
The number of any particle in (1) 1 mol is Avon Garderot constant, which does not change with temperature, pressure and other conditions.
(2) When applying Avon Gadereau's law and its inference, we should first judge whether a substance is a gas at a given temperature and pressure. If matter is not gaseous, then Evan Gardlow's law does not apply.
(3) Avon Gadereau's law is applicable to both pure gas and mixed gas. If it is a mixed gas, there can be no chemical reaction between components, such as 2NO+O2 = = 2NO2 is not applicable.
Classic training 1
(20 10? Fujian comprehensive? T7？ 6 points) indicates that avo is a Gadro constant, and the following judgment is correct.
A. 18 contains oxygen atoms. Under standard conditions, 22.4 liters of air contains one molecule.
C. 1 molCl2 The number of transferred electrons participating in the reaction must be 2.
D, dissolved in 1L water with the concentration of1mol/l.
Proposition conception This topic takes the examination of candidates' knowledge application and calculation ability as the starting point, focusing on the mastery of the calculation and judgment of Avon Gadereau constant. In recent years, the examination of this topic in the college entrance examination has maintained a certain continuity. For example, the conversion between some quantities through Avon Gadereau constant has become a hot spot in the college entrance examination.
To solve this problem, we should pay attention to two points: (1) master the conversion formula between Avon Garderot constant and other quantities; (2) Master the properties of common substances. [Source: Subject Network ZXXK]
Select a as the standard solution.
Option A: The amount of substance A: 18 is 0.5mol, so the amount of substances containing oxygen atoms is 1 mol, that is, 1NA, and A is correct; Option b: air is a mixture of various components, not just a simple substance, b is wrong; Option C: In the reaction between chlorine and water, 1 mol chlorine only transfers 1NA electrons, so item C is wrong; Option D: The amount of NA-effective sodium hydroxide substance in the obtained solution is 1 mol, but the volume of the solution is not 1L, so the quantity concentration of substances in the solution is not 1mol/L, and D is wrong.
Solve the problems that should be paid attention to in the calculation of Avon Gadereau constant by expanding the class problems;
1. The solution is N=m/M? The calculation of basic particles centered on NA should pay attention to the type of particles needed.
2. To solve this problem, we should pay attention to:
(1) If the title gives the volume of a substance, first, it depends on whether it is a standard condition. If it is not the standard condition, the volume of 1mol gas is not necessarily 22.4 L；; Second, it depends on whether it is gas. If it is not a gas, it is impossible to calculate the amount of its substance and the number of its molecules.
(2) If the topic gives the mass of gas or the quantity of substance, then the number of particles has nothing to do with external conditions.
(3) If the gas is a rare gas, it should be noted that it is a monoatomic molecule.
3. Get in touch with the basic knowledge of material structure and investigate the number of electrons, protons and neutrons contained in the material. To solve this kind of problem, it is necessary to understand the composition of matter and correctly use the calculation relationship between particles.
4. Combined with the knowledge of redox, the number of electron transfer in redox process is emphatically investigated. When solving such problems, we should grasp the essence of redox reaction and the law of electron conservation.
Second, the calculation focuses on the total amount of matter.
Dial:
What should be paid attention to in the calculation centered on the quantity of matter?
1. "One center": it must be centered on the quantity of matter. ?
2. "Two Premises": In the application of Vm=22.4 L? When mol- 1, there must be two preconditions: "standard configuration" and "gas" (mixed gas is also applicable). ?
3. "Three relationships":
(1) The relationship between particles that directly constitute matter and particles that indirectly constitute matter (atoms, electrons, etc.). ); ?
(2) the relationship between molar mass and relative molecular mass; ?
(3) The relationship between "strong, weak and non-"electrolyte and the number of solute particles (molecules or ions). ?
4. "Seven irrelevances": the quantity, quality and particle number of a substance have nothing to do with temperature and pressure; The quantity and concentration of substances have nothing to do with the volume of solution (but the quantity of solute particles is related to the volume of solution).
5. Calculation law of solution mass fraction after mixing with solute solution.
(1) When two solutions with the same solute are mixed with the same mass, the mass fraction of the mixed solution is half of the sum of the mass fractions of the two solutions. ?
(2) When two solutions with the same solute are mixed in the same volume:?
(1) For solutions with density less than 1 g/cm3, such as aqueous solution of ammonia and ethanol, when mixed in equal volume, the mass fraction is less than half of the sum of the two solutions; ?
② For solutions with density greater than 1 g/cm3, such as sulfuric acid solution and sodium chloride solution, when mixed in the same volume, the mass fraction is more than half of the sum of the mass fractions of the two solutions.
Third, the error analysis of preparing a substance concentration solution.
Principle: cB = nB/V=m/MV(M- the molar mass of solute is constant when preparing solution).
Error analysis:
1. Error caused by weighing or measurement.
The main reasons for the high concentration of substances in (1) preparation solution are:
① The balance weight is stained with other substances or rusted, resulting in the actual value of the weighed substance being greater than the weighed value;
② Forgot to return to zero when weighing;
③ When adjusting the zero point of the balance, put the stray code at the right end of the scale line;
(4) When measuring liquid with burette, look up at the beginning and look down at the end, so that the measured liquid volume becomes larger, and so on.
(2) The main reasons for the low concentration of substances in the prepared solution are:
(1) is directly weighed as hot material, which contains moisture, and the actual weight is small;
② Incomplete weight;
(3) When weighing other easily absorbed components in the air or volatile substances in an open container, the action is too slow and easy to deteriorate;
(4) When measuring liquid with burette, head-up reading should be made at the beginning and head-up reading should be made at the end, so that the measured liquid volume is small, and so on.
2. The beaker used to dissolve the diluted solution is not cleaned with distilled water, which reduces the amount of solute and leads to low solution concentration.
3. When transferring or stirring the solution, some liquid overflows, resulting in low solution concentration.
4. The temperature of the solution in the volumetric flask is higher than 20℃, resulting in the measured volume of the solution being smaller than the liquid volume marked on the volumetric flask, resulting in the high concentration of the solution.
5. When the volume of the volumetric flask is constant, reading from the bottom will increase the volume of the solution, resulting in low solution concentration; Ignoring the reading will reduce the volume of the solution, resulting in a high solution concentration.