Which of the following laws is obeyed If at constant temperature, 200cm of hydrogen at 1.5 × 10m5Nm-2pressure becomes 300cm3 at 1.0 × 105Nm-2 pressure?

Analyze the question: A gas is kept at constant temperature. Its volume changes from 200 cm³ to 300 cm³ while its pressure changes from 1.5 × 10⁵ N/m² to 1.0 × 10⁵ N/m². We need to identify which gas law this follows.

Concept: Boyle's law says that for a fixed mass of gas at constant temperature, pressure is inversely proportional to volume:

P₁V₁ = P₂V₂

Solution:

• Initial: P₁V₁ = (1.5 × 10⁵)(200) = 3.0 × 10⁷
• Final: P₂V₂ = (1.0 × 10⁵)(300) = 3.0 × 10⁷

The two products are equal, so pressure × volume stays constant at constant temperature. The pressure fell and the volume rose in the same proportion, which is exactly what Boyle's law describes.

Why the others are wrong:

• Avogadro's law relates volume to the number of moles at the same temperature and pressure. Here the amount of gas is not changing.
• Charles' law needs pressure constant while temperature changes. Here temperature is constant.
• Faraday's law belongs to electrolysis.
• Graham's law concerns the rates of diffusion of gases.

Answer: B. Boyle's law

Exam tip: When you see "constant temperature" with pressure and volume changing, think Boyle straight away. Quickly check that P × V gives the same number on both sides. Remember: Boyle = Back-and-forth (inverse) between P and V at constant T.