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A-Level Ideal gases and working with gas mixtures
What the A-Level syllabus expects for Ideal gases and working with gas mixtures, and how to practise it.
What the syllabus expects
- List the core assumptions the kinetic theory rests on when it treats a gas as ideal.
- Give a qualitative account, framed by molecular size and the forces between molecules, of:
- the conditions that nudge a real gas towards ideal behaviour
- why ideality fails once pressure is very high or temperature very low
- Quote pV = nRT and put it to work in calculations, among them the finding of Mr.
- Apply Dalton's Law to work out the partial pressure of each gas within a mixture.
Scope: see also Section 9
How it's examined
Questions on this topic most often ask you to calculate, suggest. About 3% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (3 marks)
Using Table 5.1 (helium molar mass 4.0 g/mol; air about 29.0 g/mol; tyre gas volume 2.0 dm3; pressure 8 bar; temperature 298 K), calculate the mass of helium and of air needed under identical conditions, and suggest, with a reason, whether helium gives a worthwhile mass saving.
Show the worked answer
Both gases occupy the same volume at the same P and T, so they contain the same number of moles. n = PV/RT = (8 x 10⁵ Pa x 2.0 x 10⁻³ m³)/(8.314 x 298) = 1600/2477.6 = 0.646 mol. Mass of helium = 0.646 x 4.0 = 2.6 g. Mass of air = 0.646 x 29.0 = 18.7 g. Mass saving = 18.7 - 2.6 = 16.1 g per tyre. Although helium is about 86% lighter than air, the absolute saving is only ~16 g per tyre, which is negligible compared with the mass of the tyre, wheel and vehicle (several kilograms). So it is not a worthwhile mass saving (and helium is costly and leaks readily through rubber).
Example 2 (2 marks)
Give two of the key assumptions made in the ideal gas model.
Show the worked answer
Any two of the standard kinetic-model assumptions: - The gas molecules themselves have negligible volume compared with the volume of the container. - There are no (negligible) intermolecular forces of attraction or repulsion between the molecules. - Collisions between molecules (and with the walls) are perfectly elastic, so no kinetic energy is lost. - Molecules are in constant, random motion.
Example 3 (2 marks)
Hydrazine, N2H4, behaves non-ideally as a gas. Give two factors that account for this departure from ideal-gas behaviour and account for each.
Show the worked answer
The ideal-gas model assumes gas molecules have negligible volume and exert no forces on one another. For N2H4 both assumptions fail. (1) The molecules occupy a finite/appreciable volume, which is not negligible compared with the container volume (especially at high pressure), so the available volume is less than assumed. (2) There are significant intermolecular forces between N2H4 molecules, in particular hydrogen bonding (N-H bonds and N lone pairs), so molecules attract one another rather than moving independently as an ideal gas requires.
More A-Level H2 Chemistry topics
The make-up of the atom and how its electrons are arranged · How atoms bond and how that governs a substance's behaviour · Competing definitions of acids and bases · Trends in the elements across a period and down a group · The mole and reacting-quantity calculations · Enthalpy, entropy and the feasibility of reactions · all of A-Level H2 Chemistry