Discussion
Hermetic storage of \( 20 \) tonnes in a sealed structure of \( 30 \) m\(^3\) free air space at \( 21 \) per cent oxygen contains oxygen of mass about
Answer: A
Correct answer: 8.8 kg
Take the oxygen fraction of the free air volume, then convert to mass using the density of oxygen at ambient conditions, about \( 1.4 \) kg/m\(^3\):
\[ V_{O_2} = 0.21 \times 30 = 6.3\ \mathrm{m}^3 \qquad m = 6.3 \times 1.4 \approx 8.8\ \mathrm{kg} \]
The distractor \( 6.3 \) is the volume mistaken for a mass. The point of the calculation is how little oxygen a sealed store holds: 20 tonnes of respiring grain and insects will consume that quantity within days, which is exactly how hermetic storage kills the infestation.
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