WASSCE Physics · Paper 1 style

Gas laws

One sample question with the full working. 10 more in the app, with instant feedback and repair sessions for the traps below.

A fixed mass of gas at constant temperature occupies 4 dm³ at 100 kPa. Find its pressure when the volume becomes 2 dm³.

A100 kPa
B50 kPa
C800 kPa
D200 kPa

Show the answer and working

Answer: D

Boyle: P₁V₁ = P₂V₂ ⇒ P₂ = 100×4/2 = 200 kPa.

Trap A Treats inverse relations as direct: At constant temperature PV = constant: halving the volume DOUBLES the pressure. Charles: V/T constant with T in KELVIN.

Trap B Treats inverse relations as direct: At constant temperature PV = constant: halving the volume DOUBLES the pressure. Charles: V/T constant with T in KELVIN.

Trap C Inverts a formula: Write the defining equation in words first (density = mass per unit volume), then rearrange algebraically. Check units: kg/m³.

Traps in Gas laws

Treats inverse relations as direct
At constant temperature PV = constant: halving the volume DOUBLES the pressure. Charles: V/T constant with T in KELVIN.
Inverts a formula
Write the defining equation in words first (density = mass per unit volume), then rearrange algebraically. Check units: kg/m³.
Adds quantities with different units
Only quantities with the same unit can be added. If the units differ, the formula multiplies or divides them.
Uses Celsius temperatures in gas laws
Gas laws need absolute temperature: T(K) = θ(°C) + 273. Convert every temperature before using a ratio.

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