Question

Equal volumes of H2 and O2 are placed in a balloon and then ignited. Assuming that the reaction goes to completion, which gas will remain in excess? 2H2 + O2 → 2H2O

Answer 1

Answer: Oxygen

Explanation:

$2H_2+O_2\rightarrow 2H_2O$

As seen from the balanced equation:

2 moles of hydrogen combine with 1 mole of oxygen or 1 mole of hydrogen combine with 1/2 mole of oxygen.

According to Avogadro's law, equal volumes of gases contain equal number of molecules and hence equal number of moles.

Thus if x moles of both gases are present, x moles of hydrogen will combine with $\frac{x}{2}$ moles of oxygen and  $x-\frac{x}{2}=\frac{x}{2}$ moles of oxygen will be left used and will be in excess.