Study of Gas Laws
ICSE Class 9 · Chemistry · Chapter 7
What this chapter covers
- The behaviour of gases
- Boyle's law
- Charles's law
- Absolute zero and the Kelvin scale
- The gas equation
- Standard temperature and pressure (STP)
Key points
- Gas molecules are in constant random motion, far apart, with negligible attraction; their average kinetic energy is proportional to the absolute temperature.
- Pressure is the force per unit area from molecular collisions with the walls; 1 atm = 760 mm of Hg = 101.325 kPa.
- Boyle's law: at constant temperature, V ∝ (1)/(P), so P1V1 = P2V2; graphs: V–P hyperbola, V–1/P straight line through origin, PV–P horizontal line.
- Charles's law: at constant pressure, V ∝ T (kelvin), so (V1)/(T1) = (V2)/(T2); a gas changes by (1)/(273) of its 0 °C volume per degree.
- Absolute zero is –273 °C = 0 K; T (K) = t (°C) + 273.
- Gas equation: (P1V1)/(T1) = (P2V2)/(T2); always use kelvin and matching units.
- STP: 273 K and 760 mm of Hg (1 atm); gas volumes are converted to STP for comparison.
Questions answered
State two postulates of the kinetic molecular theory of gases.
Gas molecules are in constant random motion, and the actual volume of the molecules is negligible compared with the volume of the gas; also, their average kinetic energy is proportional to the absolute temperature.
Explain, in terms of molecules, why a gas exerts pressure.
Moving molecules collide with the walls of the container and push on them; the total force of these collisions per unit area is the pressure.
State Boyle's law and write its mathematical form.
At constant temperature, the volume of a given mass of dry gas is inversely proportional to its pressure; P1V1 = P2V2.
Describe the graphs of V against P and of PV against P for a gas obeying Boyle's law.
V against P is a rectangular hyperbola; PV against P is a horizontal straight line.
600 cm3 of a gas at 1 atm is compressed to 200 cm3 at constant temperature. Find the new pressure.
P2 = (1 × 600)/(200) = 3 atm.
State Charles's law. Why must temperature be in kelvin?
At constant pressure, the volume of a given mass of dry gas is directly proportional to its absolute temperature. Volume is proportional to temperature only on the Kelvin scale, which starts at absolute zero.
A gas occupies 200 cm3 at 27 °C. Find its volume at 177 °C at constant pressure.
V2 = (200 × 450)/(300) = 300 cm3.
What is absolute zero? Why can no gas actually reach zero volume?
Absolute zero is –273 °C, the temperature at which a gas's volume would theoretically become zero. Every real gas liquefies before reaching it.
Past board exam questions
ICSE 2019 sample paper · 1 mark
The temperature of 0° Celcius on the Kelvin scale is equal to:
- –273 K
- 273 K
- 0 K
- 100 K
Show answer
273 K
ICSE 2019 sample paper · 5 marks
(i) State Boyle’s law. (ii) What is standard temperature and pressure? (iii) Calculate the percentage of nitrogen in ammonium nitrate. [NH₄NO₃] [N = 14, H = 1, O = 16]
Show answer
(i) Boyle’s law: at constant temperature, the volume of a given mass of a dry gas is inversely proportional to its pressure (PV = constant).
(ii) Standard temperature is 0 °C (273 K) and standard pressure is 760 mm (76 cm) of mercury, i.e. 1 atmosphere.
(iii) Molecular mass of NH₄NO₃ = 14 + 4(1) + 14 + 3(16) = 80
Mass of nitrogen in it = 2 × 14 = 28
Percentage of nitrogen = (28 / 80) × 100 = 35%
ICSE 2019 sample paper · 3 marks
A gas occupies 70 litres at 27°C. What volume will it occupy at 273°C, pressure remaining constant?
Show answer
V₁ = 70 L, T₁ = 27 + 273 = 300 K; T₂ = 273 + 273 = 546 K
By Charles’s law, V₁/T₁ = V₂/T₂
V₂ = (70 × 546)/300 = 127.4 litres