Ideal gases practice questions.

A fixed mass of gas occupies 3.0 dm³ at 300 K. Determine its volume at 400 K, at constant pressure.

• 2.25 dm³
• 3.0 dm³
• 4.0 dm³
• 4.5 dm³

Answer: 4.0 dm³

At constant pressure V/T is constant: V2 = V1 × T2/T1 = 3.0 × 400/300 = 4.0 dm³.

Determine the amount, in mol, of an ideal gas that occupies 4.54 dm³ at STP. (Molar volume at STP = 22.7 dm³ mol⁻¹.)

• 0.100
• 0.200
• 0.454
• 5.00

Answer: 0.200

n = V / Vm = 4.54 / 22.7 = 0.200 mol.

A sample of gas has volume 500 cm³ at 200 kPa. Determine its pressure when compressed to 250 cm³ at constant temperature.

• 100 kPa
• 250 kPa
• 400 kPa
• 500 kPa

Answer: 400 kPa

Boyle's law: P1V1 = P2V2, so P2 = (200 × 500) / 250 = 400 kPa.

Using pV = nRT, calculate the volume occupied by 0.500 mol of an ideal gas at 300 K and 100 kPa. (R = 8.31 J K⁻¹ mol⁻¹.)

• 6.24 × 10⁻³ m³
• 1.25 × 10⁻² m³
• 1.25 × 10⁻³ m³
• 2.49 × 10⁻² m³

Answer: 1.25 × 10⁻² m³

V = nRT/p = (0.500 × 8.31 × 300) / 100000 = 1246.5 / 100000 = 1.25 × 10⁻² m³ (12.5 dm³).

The temperature of a fixed volume of gas is raised from 27 °C to 327 °C. Determine the factor by which its pressure changes.

• It stays the same
• It doubles
• It is multiplied by 12
• It halves

Answer: It doubles

Convert to kelvin: 300 K → 600 K. At constant volume p/T is constant, so pressure doubles.

0.32 g of an ideal gas occupies 227 cm³ at STP. Determine its molar mass. (Molar volume at STP = 22.7 dm³ mol⁻¹.)

• 16 g mol⁻¹
• 32 g mol⁻¹
• 44 g mol⁻¹
• 64 g mol⁻¹

Answer: 32 g mol⁻¹

n = 0.227 / 22.7 = 0.0100 mol; M = mass/n = 0.32 / 0.0100 = 32 g mol⁻¹.