Discussion
At 60 °C dry bulb temperature, saturated water vapour pressure is 20 kPa and relative humidity is 20%. The corresponding absolute humidity of air at atmospheric pressure is nearest to
(Take molecular weight of water = 18.02 g.mol−1, molecular weight of air = 28.97 g.mol−1, and atmospheric pressure = 101.325 kPa)
Answer: A
The absolute humidity is about 0.026 kg water per kg dry air.
The partial pressure of the vapour is the relative humidity times the saturation pressure, and the humidity ratio follows from the molar mass ratio:
\[ \begin{aligned} p_{v} &= 0.20 \times 20 = 4\ \text{kPa} \\ H &= \frac{18.02}{28.97} \cdot \frac{p_{v}}{P - p_{v}} \\ &= 0.622 \times \frac{4}{101.325 - 4} \\ &= 0.622 \times 0.0411 = 0.0256 \end{aligned} \]
That rounds to 0.026. 0.041 is the pressure ratio \(p_{v}/(P - p_{v})\) without the molar-mass factor 0.622. 0.004 and 0.032 do not come from the correct vapour pressure of 4 kPa; using the saturation pressure of 20 kPa instead would give a much larger 0.153.
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