A fixed mass of gas occupies 92cm3 at 3°C. What will be its volume at 18C if the pressure remains constant?

Analyze the question: A fixed mass of gas at constant pressure, with the temperature changing from 3°C to 18°C. We need the new volume. This is a Charles's Law question.

Concept: Charles's Law says that at constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute (Kelvin) temperature:

V₁/T₁ = V₂/T₂

Temperature must always be converted to Kelvin: T(K) = °C + 273.

Solution:

• V₁ = 92 cm³
• T₁ = 3 + 273 = 276 K
• T₂ = 18 + 273 = 291 K
• V₂ = ?

V₂ = V₁ × (T₂ / T₁)

V₂ = 92 × (291 / 276)

Since 92 × 3 = 276, we get 276/92 = 3, so:

V₂ = 291 / 3 = 97.0 cm³

Answer: The volume rises slightly because the gas is heated, so the answer must be larger than 92 cm³. That rules out A, B and C, and leaves D (97.0 cm³).

Tip: Always convert to Kelvin before using any gas law. If you forget, you'll get a wrong answer (for example, using 18/3 would give 552 cm³). You can also use a quick check: if the gas is heated, the volume must increase at constant pressure, so you can eliminate options smaller than the original volume.