IB Physics SL B.3.5 Ideal Gas Equations

Practise selecting PV = nRT or PV = NkBT, converting every quantity to SI and rearranging for pressure, volume, temperature, moles or particle number in multistep contexts.

Exam points

choose nR for amount in moles or NkB for particle number without mixing the two forms
convert pressure, volume and temperature to Pa, m³ and K before substituting
use ratios or full equations to find percentage changes when more than one state variable changes

[Maximum number: 2]

The equipment shown in the diagram was used by a student to investigate the variation with volume, of the pressure p of air, at constant temperature. The air was trapped in a tube of constant cross-sectional area above a column of oil.

Figure for Question B.3.5—Ideal gas equations question 1 — IB Physics SL

The pump forces oil to move up the tube decreasing the volume of the trapped air.

The cross-sectional area of the tube is $1.3 \times 10^{-3} \mathrm{~m}^{2}$ and the temperature of air is 300 K . Estimate the number of moles of air in the tube.

$n=\frac{b A}{R T} OR$ correct substitution of one point from the graph

$n=\frac{5 \times 10^{4} \times 1.3 \times 10^{-3}}{8.31 \times 300}=0.026 \approx 0.03$

Answer must be to 1 or 2 SF. Allow ECF from (b).

All question bank results loaded

B.3.5—Ideal gas equations question 1