At constant pressure, heating a gas from 25°C to 100°C changes its volume by what factor?

• A. 1.12
• B. 1.30
• C. 1.25
• D. 1.40

Answer: (C)

At constant pressure, the volume of a gas is directly proportional to its absolute temperature, V ∝ T, for a fixed amount of gas. This comes from PV = nRT, so with P and n fixed, V scales with T when T is in kelvin.

Convert the temperatures to kelvin: 25°C is 298 K, and 100°C is 373 K. The volume ratio is V2/V1 = T2/T1 = 373/298 ≈ 1.252, which rounds to about 1.25.

So heating the gas from 25°C to 100°C at constant pressure increases its volume by roughly a factor of 1.25. Using kelvin is essential because the Celsius scale does not represent proportional temperature changes for gas-volume relationships.