4 high-yield, syllabus-aligned questions on Gas laws & kinetic theory, covering 12 individually creditable mark points. Each one shows the answer that scores full marks under our guide, which phrase earns which mark, and a common incomplete answer — so you can see the difference rather than guess at it.
Free to read. No account needed for anything on this page.
Every question and solution is reviewed by examiners versed in WAEC and JAMB, each with more than thirty years of experience.
Mark allocations are TopMarks’ own, written against the published syllabus and Chief Examiners’ reports. They are a guide to how these answers are usually credited, not an official marking scheme.
State2 marks
State Boyle's law and Charles's law.
TopMarks model answer
Boyle's law states that at constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure. Charles's law states that at constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature.
Suggested marking guide
1Boyle's law with the constant-temperature condition
2Charles's law with the constant-pressure condition and absolute temperature
Why it scores
The CONDITION is part of the law, not a preamble to it. “Volume is inversely proportional to pressure” is false unless the temperature is constant and the mass fixed, so an answer without the condition states something untrue and cannot be credited. Charles’s law needs one more word still: ABSOLUTE temperature, in kelvin.
A common incomplete answer
“Boyle's law says volume is inversely proportional to pressure, and Charles's law says volume is proportional to temperature.”
What it costs: Both marks. Neither condition is stated, and Charles’s law is given with temperature rather than ABSOLUTE temperature — with Celsius the relationship does not hold at all.
A gas occupies 250 cm³ at 27 °C and 760 mmHg. Calculate its volume at s.t.p.
TopMarks model answer
Convert to kelvin: T₁ = 27 + 273 = 300 K. At s.t.p. T₂ = 273 K and P₂ = 760 mmHg. Using the general gas equation P₁V₁ ÷ T₁ = P₂V₂ ÷ T₂: 760 x 250 ÷ 300 = 760 x V₂ ÷ 273. Therefore V₂ = 250 x 273 ÷ 300 = 227.5 cm³.
Suggested marking guide
1Temperatures converted to kelvin
2General gas equation stated
3Correct substitution of the pressures and volume
4Correct answer with the unit
The working, line by line
1T₁ = 27 + 273 = 300 K — Gas-law temperatures are ALWAYS in kelvin. Using 27 directly is the commonest single error in the topic.
2At s.t.p.: T₂ = 273 K, P₂ = 760 mmHg — Know the standard conditions by heart — 273 K and 760 mmHg (or 1 atm, or 101325 Pa).
3P₁V₁ ÷ T₁ = P₂V₂ ÷ T₂ — Write the equation before substituting. It is a method mark on its own.
4760 x 250 ÷ 300 = 760 x V₂ ÷ 273 → V₂ = 227.5 cm³ — The pressures cancel here, but only because they happen to be equal — show them anyway so the examiner sees you checked.
Why it scores
The pressure in this question is the same before and after, so it cancels. That is a trap: a candidate who notices the cancellation and never writes the pressures down forfeits the substitution mark, because the examiner cannot tell whether it was cancelled deliberately or ignored.
A common incomplete answer
“V₂ = 250 x 273 ÷ 300 = 227.5 cm³”
What it costs: Two marks of four. The kelvin conversion and the answer are both credited, but the general gas equation is never stated and the pressures never appear.
Key ideas to include: kelvin, s.t.p., general gas equation, 273 K, 760 mmHg.
State3 marks
State three assumptions of the kinetic theory of gases.
TopMarks model answer
A gas consists of a very large number of tiny particles which are in continuous, random motion. The volume of the particles themselves is negligible compared with the volume of the container, so most of the gas is empty space. There are no attractive or repulsive forces between the particles, and all collisions are perfectly elastic, so no kinetic energy is lost in a collision.
Suggested marking guide
1Particles in continuous random motion
2Volume of the particles is negligible
3No intermolecular forces and perfectly elastic collisions
Why it scores
These assumptions are also the answer to “why do real gases deviate from ideal behaviour?” — because at high pressure the particle volume stops being negligible, and at low temperature the forces stop being absent. Learning the assumptions gives you both questions for the price of one.
A common incomplete answer
“Gas particles move about randomly in all directions.”
What it costs: Two marks of three. Correct, but the negligible particle volume and the perfectly elastic collisions — the two assumptions that actually do the work — are missing.
Key ideas to include: random motion, negligible volume, no intermolecular forces, perfectly elastic collisions.
Explain3 marks
Explain why a gas exerts pressure on the walls of its container, and why that pressure increases when the gas is heated at constant volume.
TopMarks model answer
The particles of a gas are in continuous random motion and collide with the walls of the container. Each collision exerts a force on the wall, and the total force acting per unit area of the wall is the gas pressure. When the gas is heated at constant volume the particles gain kinetic energy and move faster, so they collide with the walls more frequently and with greater force, and the pressure therefore increases.
Suggested marking guide
1Particles collide with the walls
2Pressure is the force per unit area
3Heating raises the speed and the frequency of collisions
Why it scores
The word FREQUENCY carries the third mark. Heating does two things — the particles hit harder AND they hit more often — and an answer that mentions only speed has given half the mechanism. Examiners look for the collision rate specifically.
A common incomplete answer
“Because the particles hit the walls of the container.”
What it costs: Two marks of three. The collisions are credited, but pressure is never linked to force per unit area, and the effect of heating is not explained at all.
Key ideas to include: collision, force per unit area, kinetic energy, frequency of collision.
Practise Gas laws & kinetic theory on real questions
Reading a full-mark answer is the first half. Writing one under time is the other. A free account opens exam-standard practice in Chemistry with the full solution on every question.