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11/09/2020 12:18 pm
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The vapour pressure of water is 12.3 kPa at 300 K. Calculate vapour pressure of 1 molal solution of a non-volatile solute in it.
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11/09/2020 12:21 pm
The vapour pressure of water is 12.3 kPa at 300 K. We need to calculate the vapour pressure of 1 molal solution of a non-volatile solute in it.
1000 g of water contains 1 mole of solute.
Molar mass of water is 18 g/mol.
Number of moles of water = 1000/18 = 55.56 mol
Mole fraction of the solute in the solution is x = 1 /1 + 55.56 = 0.0177.
The relative lowering in the vapour pressure is equal to the mole fraction of solute.
p0 - p /p0 = x (12.3 - p)/12.3 = 0.0177 ⟹ p = 12.08
Hence, the vapour pressure of the solution is 12.08 kPa.