The volume of a given mass of gas is 40cm3 at 27° C. What is its volume at 90° C if its pressure remains constant?

Analyze the question

A fixed mass of gas is heated at constant pressure. This is a Charles's Law problem.

Concept

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

V₁/T₁ = V₂/T₂

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

Solution

• V₁ = 40 cm³
• T₁ = 27 + 273 = 300 K
• T₂ = 90 + 273 = 363 K
• V₂ = ?

V₂ = V₁ × (T₂ / T₁) V₂ = 40 × (363 / 300) V₂ = 40 × 1.21 V₂ = 48.4 cm³

Answer

The volume at 90 °C is 48.40 cm³, which matches option C.

Tip

In gas law questions, always convert to Kelvin first. If you forget and use Celsius directly (40 × 90/27 ≈ 133 cm³), you will get a value that is not among the options. Also remember that when temperature rises at constant pressure, volume must rise, so the answer must be greater than 40 cm³. This lets you quickly eliminate A and B.