CS_2 (g) + 3O_2(g) -----> CO_2(g) + 2SO_2(g)?

Question

A 20 cm cube sample of carbon disulphide vapour is ignited with 100 cm cube of oxygen. The final volume of gas after burning is treated with an excess of aqueous alkali.
What percentage of this final volume dissolves in the alkali?

Answer 1

60%, here's how:
1 mole(22.4 dm3) of CS2 reacts with 3 moles (3*22.4dm3) of O2 so 20 cm3 of CS2 will react with x=20*3*22.4/22.4=60 cm3 O2.
You'll be left with 40 cm3 of unreacted oxygen
now when you do the same calculation for the products you'll find that you'll make 20 and 40 cm3 of CO2 and SO2, respectively, so the final volume is 20+40+40=100 cm3, of that 60 cm3 is base soluble (both CO2 and SO2 react with a base and dissolve very well) and thus the 60%

Answer 2

20ml of CS2 requires 60ml of O2. Thus there are 40 spare ml of oxygen. The product gases take up 60ml, so the total is 100ml, and the acidic gases therefore occupy 60% of the final mixture.