What volume of oxygen will remain after reacting 8cm$^3$ of hydrogen gas with 20cm$^3$ of oxygen gas?

Analyze the question: Hydrogen and oxygen react to form water. We need to find which gas is limiting and how much oxygen is left over.

Concept: Gay-Lussac's law of combining volumes. Gases react in simple whole-number ratios by volume (at the same temperature and pressure).

Balanced equation:

2H₂(g) + O₂(g) → 2H₂O

So 2 volumes of H₂ react with 1 volume of O₂.

Solution:

1. Oxygen needed to react with 8 cm³ of H₂: (1/2) × 8 = 4 cm³ of O₂

2. Oxygen available = 20 cm³, so oxygen is in excess and hydrogen is the limiting reactant.

3. Oxygen remaining: 20 − 4 = 16 cm³

(The water formed is liquid at room temperature, so it takes up negligible volume. Only the leftover oxygen counts.)

Answer: The remaining oxygen is 16 cm³, which is option D.

Tip: For gas-volume questions, use the mole ratio from the balanced equation directly as a volume ratio. Always find the limiting reactant first, then subtract what was used from the excess gas.