(2) According to the experimental requirements, the best device in box B is analyzed: device A adopts drainage method to collect, and no dry oxygen can be obtained; Device B can collect pure and dry oxygen and observe the amount of gas collected; When equipment c collects oxygen, the air in the gas cylinder cannot be excluded, so the answer is: B.
(3) When oxygen is collected according to the fact that oxygen is insoluble in concentrated sulfuric acid, bubbles begin to appear. Don't collect them immediately, because the air mixed with them is impure. Only when bubbles emerge uniformly from A can they be collected. Therefore, the answer is: see bubbles emerge evenly from A.
(4) After the experiment, students found that the gas collected was much less than expected. This phenomenon may be: device A leaks air, balloon leaks air, and the dosage is small, so the answer is: device A leaks air (or balloon leaks air) and so on.
(5) When recovering the water-insoluble manganese dioxide from the reaction residue, the potassium chloride (or potassium chlorate which is not completely reacted) in the residue can be dissolved in water, so the operation steps are: dissolution, filtration and drying. So, the answer is: filtering.
So the answer is:
(1) concentrated sulfuric acid?
(2)b
(3) See bubbles emerge from A evenly.
(4) Equipment A leaks air (or balloon leaks air)
(5) filtering