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IBDP Physics- B.3 Gas laws- IB Style Questions For SL Paper 1A -FA 2025

IBDP Physics SL Paper 1 -FA 2025- All Chapters

Question

An ideal gas is held in a cylinder by a piston as shown in picture below. The piston compresses the gas rapidly.
Which one of below is correct .
The average speed of the gas molecules increases because the gas molecules…
(A) have a smaller volume available in which they can move.
(B) receive thermal energy transferred from outside the cylinder.
(C) receive energy from the piston as they collide with it.
(D) make more collisions every second with each other.
▶️ Answer/Explanation
Detailed solution

1. Analyze Process:
Rapid compression implies an adiabatic process (no time for heat transfer, so Q=0). The work is done on the gas.

2. Microscopic View:
When a molecule collides with a moving wall (the piston moving inward), it rebounds with a higher speed than it struck with (similar to a tennis ball hit by a racket). This gain in kinetic energy increases the average speed (and temperature) of the gas.
✅ Answer: (C)

Question

A balloon of volume \(V\) contains \(10\,\text{mg}\) of an ideal gas at a pressure \(P\). An additional mass of the gas is added without changing the temperature of the balloon. This causes the volume to increase to \(2V\) and the pressure to increase to \(3P\).
What is the mass of gas added to the balloon?
A. \(5\,\text{mg}\)
B. \(15\,\text{mg}\)
C. \(50\,\text{mg}\)
D. \(60\,\text{mg}\)
▶️ Answer / Explanation

At constant temperature, the ideal gas law gives \(PV \propto n\).

Initial state:
\(PV = nRT\), where the mass is \(10\,\text{mg}\).

Final state:
Pressure \(=3P\), volume \(=2V\), and total mass \(=(10+m)\,\text{mg}\).

Using proportionality:
\( \frac{3P \times 2V}{PV} = \frac{10+m}{10} \)
\( 6 = \frac{10+m}{10} \Rightarrow 10+m = 60 \Rightarrow m = 50\,\text{mg} \)

✅ Answer: C

Question

The pressure of a fixed mass of an ideal gas in a container is decreased at constant temperature. For the molecules of the gas there will be a decrease in
A. the mean square speed.
B. the number striking the container walls every second.
C. the force between them.
D. their diameter.
▶️ Answer / Explanation

At constant temperature, the average kinetic energy of molecules is constant, so the mean square speed does not decrease.

A lower pressure means a lower rate of momentum transfer to the container walls, which happens when fewer molecules collide with the walls each second.

Therefore, the quantity that decreases is the number striking the container walls every second.

✅ Answer: B

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