What is an ideal gas?

It is a gas whose particles occupy negligible volume and exert no forces on one another except during collisions, so that pV = NkT holds at all pressures and temperatures.

Describe the shape of a graph of pressure against volume for a fixed mass of ideal gas at constant temperature.

It is a curve that falls steeply and then flattens, a hyperbola, along which the product of the coordinates stays the same.

How does the volume of a fixed mass of gas at constant pressure depend on its temperature?

The volume is directly proportional to the absolute temperature.

A gas at 1.0 × 10⁵ Pa occupies 250 cm³ and is compressed at constant temperature to 100 cm³. Calculate the new pressure.

The new pressure is 1.0 × 10⁵ × 250 ÷ 100 = 2.5 × 10⁵ Pa.

OCR A-Level Physics B H557

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Matter: Very Simple

Ideal gases and the gas laws

3 things to know
Components 01, 02 and 03

Overview

Recall questions on ideal gases, the pressure–volume and volume–temperature relationships and their graphs, p against 1/V, compression and heating calculations, pressure explained by particle collisions, absolute temperature and converting to kelvin, when real gases depart from ideal behaviour and why they condense, and pV against p and p against T graphs.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Explain in terms of particles why compressing a gas at constant temperature raises its pressure.

Key term

State the relationship between the pressure and volume of a fixed mass of gas held at constant temperature.

Key term

Which graph turns pressure and volume data into a straight line, and what does its gradient equal?

Questions from this subtopic

Each one with the model answer underneath.

1
What is an ideal gas?
Model answer
It is a gas whose particles occupy negligible volume and exert no forces on one another except during collisions, so that pV = NkT holds at all pressures and temperatures.
2
State the relationship between the pressure and volume of a fixed mass of gas held at constant temperature.
Model answer
The pressure is inversely proportional to the volume, so their product is constant.
3
Describe the shape of a graph of pressure against volume for a fixed mass of ideal gas at constant temperature.
Model answer
It is a curve that falls steeply and then flattens, a hyperbola, along which the product of the coordinates stays the same.
4
Which graph turns pressure and volume data into a straight line, and what does its gradient equal?
Model answer
Pressure plotted against the reciprocal of volume gives a straight line through the origin with gradient NkT.
5
How does the volume of a fixed mass of gas at constant pressure depend on its temperature?
Model answer
The volume is directly proportional to the absolute temperature.
6
A gas at 1.0 × 10⁵ Pa occupies 250 cm³ and is compressed at constant temperature to 100 cm³. Calculate the new pressure.
Model answer
The new pressure is 1.0 × 10⁵ × 250 ÷ 100 = 2.5 × 10⁵ Pa.

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