In the reaction 2H₂(g) + O₂(g) → 2H₂O(g), what volume of hydrogen would be left over when 300 cm³ of oxygen and 1000 cm³ of hydrogen are exploded in a sealed tube?

Correct answer: 400 cm³ (Option B)

Explanation: For gaseous reactions at the same temperature and pressure, volumes combine in the same ratio as moles (Gay-Lussac’s law). The reaction is:

\(2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(g)\)

• Required ratio H₂ : O₂ = 2 : 1.
• Given O₂ = 300 cm³ ⇒ H₂ needed = \(2 \times 300 = 600\) cm³.
• Available H₂ = 1000 cm³ ⇒ excess H₂ = \(1000 - 600 = \mathbf{400}\) cm³.

Why the others are incorrect: 200 cm³ and 700 cm³ come from subtracting the wrong amounts; 600 cm³ would be the H₂ consumed, not left over.