IB DP Chemistry - Structure 1.5 Ideal gases - IB Style Questions For HL Paper 1A -FA 2025

IBDP Chemistry HL Paper 1A & 1B - All Chapters

Question 

A fixed volume of an ideal gas, initially at \(30^\circ\mathrm{C}\) and with a pressure of \(P\,\mathrm{Pa}\), was heated to \(60^\circ\mathrm{C}\).

What is the resulting pressure in \( \mathrm{Pa} \)?

(A) Half of \(P\)
(B) A little less than \(P\)
(C) A little more than \(P\)
(D) Two times \(P\)
▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathrm{C}} \)

For a fixed volume of an ideal gas, pressure is directly proportional to absolute temperature:

\( \dfrac{P_1}{T_1}=\dfrac{P_2}{T_2} \)

Convert the temperatures to kelvin:

\(T_1=30+273=303\,\mathrm{K}\)

\(T_2=60+273=333\,\mathrm{K}\)

Therefore:

\(P_2=P\times\dfrac{333}{303}\approx1.10P\)

The pressure increases by about \(10\%\), so it is a little more than \(P\).

Therefore, the correct answer is (C).

Question 

In which of the following sets of conditions does the gas exhibit behaviour closest to an ideal gas?

OptionGasPressure / \( \mathrm{kPa} \)Temperature / \( \mathrm{K} \)
A\( \mathrm{H_2} \)\(100\)\(273\)
B\( \mathrm{NH_3} \)\(50\)\(473\)
C\( \mathrm{H_2} \)\(50\)\(473\)
D\( \mathrm{NH_3} \)\(100\)\(273\)
▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathrm{C}} \)

A gas behaves most like an ideal gas at low pressure and high temperature.

At low pressure, gas particles are further apart, so intermolecular forces and the volume occupied by the particles become less significant. At high temperature, the particles have greater kinetic energy and are less affected by intermolecular attractions.

Hydrogen, \( \mathrm{H_2} \), also has relatively weak intermolecular forces compared with ammonia, \( \mathrm{NH_3} \).

Therefore, the conditions in option (C), \(50\,\mathrm{kPa}\) and \(473\,\mathrm{K}\) for \( \mathrm{H_2} \), give behaviour closest to that of an ideal gas.

Question

The volume \(V\) for a fixed mass of an ideal gas was measured at constant temperature at different pressures \(p\). Which graph shows the correct relationship between \(pV\) against \(p\)?
graphs of pV versus p for an ideal gas
▶️ Answer/Explanation
Detailed solution

For a fixed mass of an ideal gas at constant temperature, Boyle’s Law states that:
\[ pV = \text{constant} \]

Since the product \(pV\) is constant, its value does not change as \(p\) changes. Therefore, a graph of \(pV\) (vertical axis) against \(p\) (horizontal axis) must be a horizontal line.

Among the given graphs, the only one that shows a constant value of \(pV\) as \(p\) increases is option (A).
✅ Answer: (A)

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