Gas laws — KCSE Physics

KCSE Physics · 111 practice questions · 6 syllabus objectives

31 easy41 medium39 hard

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

State the basic assumptions of the kinetic theory of gases and explain gas laws using the kinetic model

Solve numerical problems involving gas laws

State and verify experimentally Boyle's law, Charles' law and pressure law

Explain how absolute zero temperature may be obtained and convert between Celsius and Kelvin scales

Apply the combined gas law P₁V₁/T₁ = P₂V₂/T₂ to solve problems involving simultaneous changes in P, V and T

State applications of gas laws and explain real-world phenomena using gas behaviour

Sample Questions

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1
easySHORT ANSWER3 marks

Identify how the behaviour of gas in a sealed container changes with temperature increase. (3 marks)

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Part (a) — 3 marks
Gas molecules gain kinetic energy and move faster (1 mk)
Increased molecular motion leads to higher pressure if volume is constant (1 mk)
Gas expands if the container can change volume, maintaining pressure balance (1 mk)
2
easySHORT ANSWER2 marks

Identify two everyday applications of the ideal gas law. (2 marks)

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Part (a) — 2 marks
Inflating car tyres to ensure proper pressure for safety and performance (1 mk)
Calculating the amount of gas needed for cooking in gas cylinders (1 mk)
3
easySHORT ANSWER4 marks

A sample of helium gas is contained in a sealed cylinder at a pressure of 200 kPa and a temperature of 350 K. If the pressure is reduced to 100 kPa while the temperature is decreased to 250 K, determine the initial volume of the gas if it is known that the final volume is 1.5 m³. (4 marks)

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Part (a) — 4 marks
Use the combined gas law P₁V₁/T₁ = P₂V₂/T₂ (1 mk)
Substituting known values: (200 kPa × V₁) / 350 K = (100 kPa × 1.5 m³) / 250 K (1 mk)
Rearranging to find V₁ gives V₁ = 1.125 m³ (1 mk)
Final answer with correct units: 1.125 m³ (1 mk)
4

A gas occupies a volume of 2.0 m³ at a pressure of 100 kPa and a temperature of 300 K. If the volume is increased to 4.0 m³ while the temperature rises to 600 K, calculate the final pressure of the gas. (3 marks)

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