Chemical Mass

a)
Gas product A is sulphur dioxide, which can change acidified potassiumdichromate solution from orange to green.
Gas product B is carbon dioxide, which can turn lime water milky.

b)
2ZnS + 3O2 → 2ZnO + 2SO2
2ZnO + C → 2Zn + 2CO2

c)
When the mixture is strongly heated :
2ZnS + 3O2 → 2ZnO + 2SO2
No. of moles of SO2 formed = 0.52/(32.1 + 16x2) = 0.0081 mol
No. of moles of ZnS = 0.0081 mol

When the residue is heated strongly with excess carbon :
2ZnO + C → 2Zn + CO2
No. of moles of CO2 = 0.70/(12 + 16x2) = 0.0159 mol
Total no. of moles of ZnO = 0.0159 x 2 = 0.0318 mol

Part of the ZnO is converted from the ZnS in the mixture in mole ratio 1 : 1.
No. of moles of ZnO in the mixture = 0.0318 - 0.0081 = 0.0237 mol
Mass of ZnO in the mixture = 0.0237 x (65.4 + 16) = 1.93 g

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Hint:
a. sulfur dioxide and then carbon dioxide
b. not difficult

2012-09-01 03:22:47 補充：
c.
from mass of sulfur dioxide, calc. no. of mole of zinc sulfide and thus its mass.
then, do similarly for carbon dioxide for zinc oxide.
then you get the %.