2018 H2 Chem P1 Q7 — Gases Solution

The answer is C (5%).
At sea level, partial pressure of \(\text{O}_2 = \frac{1}{5} \times 1\text{ bar} = 0.200\text{ bar}\).
The diver must inhale \(\text{O}_2\) at the same partial pressure, 0.200 bar, from a gas mixture at a total pressure of 4 bar.
Partial pressure = mole fraction \(\times\) total pressure, so mole fraction of \(\text{O}_2 = \dfrac{0.200}{4} = 0.05\).
Percentage of \(\text{O}_2\) in the tank \(= 0.05 \times 100\% = 5\%\).
• A is wrong: 0.05 is the mole fraction of oxygen; it has not been multiplied by 100 to turn it into a percentage.
• B is wrong: 0.8 comes from multiplying 0.200 by 4 instead of dividing, so it is not the fraction of the tank gas that is oxygen.
• D is wrong: 20% is the percentage of oxygen in air. At 4 bar it would give an oxygen partial pressure of \(0.20 \times 4 = 0.8\text{ bar}\), four times the sea-level value.