If 0.40 mol of H₂ and 0.15 mol of O₂ were to react as completely as possible to produce H₂O, what mass of reactant would remain?

0.1 mole of H2, that in grams will represent: 1 mol = 2 grams H2,

0.1 moles = 0.2 grams of H2

Explanation:

The complete reaction will it be:

2 H2 + O2 → 2 H2O

1. - That means that for 1 mole of O2 we produce 2 moles of H2O but we just have 0.15 moles of O2 so if we apply a rule of three:

moles of H2O to produce = (0.15 x 2) / 1 = 0.3 moles produced of H2O

2. - So as we can observe in the chemical reaction, you need 2 moles of H2 to produce 2 moles of H2O but you can just produce 0.3 moles, so again if you do a rule of three:

moles of H2 that are going to be used: (0.3 x 2) / 2 = 0.3 moles of H2

3. - Since you have 0.4 moles, but you are just going to use 0.3, you left 0.1 moles