The vapour pressure of water is 12.3kPa at 300 K. Calculate vapour pressure of 1 molal solution of a non-volatile solute in it.


Given: 1 molal solution means 1 mole of solute present in 1000g of water solvent)

Molecular weight of water = H2O = 1 × 2 + 16 = 18g/mol


No. of moles of water, n = given mass /molecular weight


n = 1000/18 = 55.56 gmol-1


Mole fraction of solute in solution,x2 = moles of solute/(moles of solute + moles of water)


x2 = 1/(1 + 55.56)


x2 = 0.0177


Given vapour pressure of pure water at 300k is 12.3 kpa


Apply the formula:




P1 = 12.0823kpa


which is the vapour pressure of the solution.


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