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

IBDP Physics HL Paper 1 -FA 2025- All Chapters

Question

A gas is contained in a vessel. A second, identical vessel contains the same number of molecules of another gas whose molecules are more massive, and both gases are at the same temperature. Which statement correctly compares the density and pressure in the two containers?
(A) same density, same pressure
(B) same density, different pressure
(C) different density, same pressure
(D) different density, different pressure
▶️ Answer/Explanation
Detailed solution

Same number of molecules + same container volume ⇒ same number density.

Pressure depends only on number of molecules and temperature, not molecular mass ⇒ same pressure.

More massive molecules ⇒ greater mass per volume ⇒ higher density. So densities differ.

Correct: different density, same pressure.
✅ Answer: (C)

Question

A balloon of volume \(V\) contains \(10\,\mathrm{mg}\) of an ideal gas at a pressure \(P\). Additional gas is added while the temperature of the balloon remains constant. As a result, the volume increases to \(2V\) and the pressure increases to \(3P\).

What is the mass of gas added to the balloon?

(A) \(5\,\mathrm{mg}\)
(B) \(15\,\mathrm{mg}\)
(C) \(50\,\mathrm{mg}\)
(D) \(60\,\mathrm{mg}\)
▶️ Answer/Explanation
Detailed solution

For an ideal gas at constant temperature, \[ PV = nRT \]

Initial state: \[ PV = \left(\frac{10}{M}\right)RT \]

Final state: \[ (3P)(2V) = \left(\frac{10 + m}{M}\right)RT \]

Dividing the final equation by the initial equation: \[ 6 = \frac{10 + m}{10} \]

Solving: \[ 10 + m = 60 \quad \Rightarrow \quad m = 50\,\mathrm{mg} \]

Correct: additional mass of gas is \(50\,\mathrm{mg}\).
✅ Answer: (C)

Question

A fixed mass of an ideal gas expands slowly at constant temperature in a container.
Three statements about the gas molecules during the expansion are:
I. They collide with the walls of the container at a reduced rate.
II. They travel further on average between each collision.
III. Their average kinetic energy decreases as the gas expands.
Which statements are correct?
(A) I and II only
(B) I and III only
(C) II and III only
(D) I, II and III
▶️ Answer/Explanation
Detailed solution

During a slow expansion at constant temperature, the average kinetic energy of gas molecules depends only on temperature and therefore remains constant.

I. As the gas expands, the number density of molecules decreases. This leads to fewer collisions with the container walls per unit time, so the collision rate is reduced.

II. With increased volume, the average distance between molecules increases. Hence, the mean free path becomes longer and molecules travel further between collisions.

III. This statement is incorrect because the temperature remains constant. Since average kinetic energy is proportional to temperature, it does not decrease.

Therefore, statements I and II are correct.
✅ Answer: (A)

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