What is the value of the gas constant R?

8.31 J K⁻¹ mol⁻¹ (given in the exam).

Convert 250 cm³ to m³.

250 × 10⁻⁶ = 2.50 × 10⁻⁴ m³.

What happens to p if T doubles at constant n and V?

p doubles, because p = nRT/V so p is proportional to T.

How do you find Mr of a volatile liquid using the ideal gas equation?

Find n = pV/RT for the vapour, then Mr = mass / n.

What is the most common error in an ideal gas calculation?

Not converting to SI units, e.g. using dm³ or cm³ instead of m³, or °C instead of K.

What is the unit of R, and what does it tell you?

J K⁻¹ mol⁻¹: it only works with SI units for p, V and T.

The ideal gas equationAQA A-Level Chemistry: Flashcards

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Write the ideal gas equation, defining each symbol.

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Write the ideal gas equation, defining each symbol.: pV = nRT: p pressure (Pa), V volume (m³), n amount (mol), R gas constant, T temperature (K).
What is the value of the gas constant R?: 8.31 J K⁻¹ mol⁻¹ (given in the exam).
What SI unit must pressure be in for pV = nRT?: Pascals, Pa (1 kPa = 10³ Pa).
Convert 25.0 dm³ to m³.: 25.0 × 10⁻³ = 0.0250 m³.
Convert 250 cm³ to m³.: 250 × 10⁻⁶ = 2.50 × 10⁻⁴ m³.
Convert 27 °C to kelvin.: 27 + 273 = 300 K.
Rearrange pV = nRT to give n.: n = pV/RT
Rearrange pV = nRT to give T.: T = pV/nR
What happens to p if T doubles at constant n and V?: p doubles, because p = nRT/V so p is proportional to T.
How do you find Mr of a volatile liquid using the ideal gas equation?: Find n = pV/RT for the vapour, then Mr = mass / n.
What is the most common error in an ideal gas calculation?: Not converting to SI units, e.g. using dm³ or cm³ instead of m³, or °C instead of K.
What is the unit of R, and what does it tell you?: J K⁻¹ mol⁻¹: it only works with SI units for p, V and T.

Exam questions on The ideal gas equation

A sample of nitrogen gas is held in a rigid flask of volume 250 cm³. The gas is at a temperature of 27 °C and a pressure of 120 kPa.
Calculate the amount, in mol, of nitrogen in the flask. The gas constant, R = 8.31 J K⁻¹ mol⁻¹.2 marks
A sealed, rigid steel cylinder of fixed volume contains helium used to fill party balloons. The cylinder is stored in a warehouse where the temperature changes between night and day.
Use the ideal gas equation to explain why the pressure in the cylinder is higher on a hot day than at night.2 marks
A student determines the relative molecular mass of a volatile liquid. She injects 0.223 g of the liquid into a gas syringe held in an oven at 100 °C. The liquid vaporises completely and the vapour occupies 95.0 cm³ at a pressure of 101 kPa. The gas constant, R = 8.31 J K⁻¹ mol⁻¹.
Calculate the amount, in mol, of vapour in the gas syringe.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).