One litre of water is placed in a closed room $$2 \times 10^4$$ liter capacity containing dry air at $$27^{\circ}\mathrm{C}$$. If aqueous tension at $$27^{\circ}\mathrm{C}$$ is $$23\mathrm{mm}$$ of $$\mathrm{Hg}$$ and density of water is $$1\mathrm{g/mL}$$, then volume of water that will evaporate is
A$$600\mathrm{mL}$$
B$$442.8\mathrm{mL}$$
C1 L
D$$200\mathrm{mL}$$
Answer & Solution
Correct answer: B. $$442.8\mathrm{mL}$$
At equilibrium, water vapour in the room will have partial pressure equal to the aqueous tension.
$$P = \frac{23}{760}\,\mathrm{atm}$$
Room volume is
$$V = 2 \times 10^4\,\mathrm{L}$$
Temperature is
$$T = 300\,\mathrm{K}$$
Using the ideal gas equation for water vapour,
$$n = \frac{PV}{RT}$$
$$n = \frac{\left(\frac{23}{760}\right)\left(2 \times 10^4\right)}{(0.0821)(300)}$$
$$n \approx 24.56$$
Mass of water evaporated is
$$m = 24.56 \times 18 \approx 442.08\,\mathrm{g}$$
Since density of water is $$1\,\mathrm{g/mL}$$, the evaporated volume is
$$442.08\,\mathrm{mL}$$
This matches option $$442.8\mathrm{mL}$$ most closely.
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