Hydrogen burns to give water. If 200cm3 of H2 reacts with 150cm3 of O2 what volume of water vapor is produced

Answer 1

2H2 + O2 ==> 2H2O
When working with gases one need not go through mols but may use volume directly.
Convert 200 L H2 to H2O using the coefficients in the balanced equation.
200 cc H2 x (2 mols H2O/2 mol H2) = 200 cc H2O

Convert 150 cc O2 to H2O.
150 cc O2 x (2 mols H2O/1 mol O2) = 3-- cc H2O.

You note that these numbers don't agree which means one of them is wrong. The correct value in limiting reagent problems (and this is a limiting reagent problem because amounts are given for BOTH reactants) is ALWAYS the smaller value. Therefore, you can produce 200 cc H2O this way.