Calculate the vapor pressure, in mm hg, at 25c of water above a solution made by dissolving 100. g sucrose, (fw = 342.3) in 250. g of water. the vapor pressure of pure water at 25c is 23.8 mm hg.

We need two pieces of information to calculate the reduction of the vapor pressure of the solvent in a solution containing a nonvolatile nonelectrolyte:

Psoln = Psolvent (1- Xsolute)

Xsolute = (100g /342.3 g/ mol) / ( 100g /342.3 g/ mol) + (250 g/ 18 g/ mol)

X solute = 0.0206

P soln = (23.8 mmHg) (1- 0.0206)

Psoln = 23.31 mmg Hg

Vapour pressure of the solution =23.31mmHg

Explanation:

Using Raoult's law: Pi = Pi × Xi

To calculate number of moles of H2O = 250g of H2O × 1 / 18g of H20 = 13.9 moles of H2O.

To calculate number of moles of sucrose= 100g of Sucrose × 1/342.3g of sucrose = 0.29moles of Sucrose.

To calculate Vapour pressure,we find:

Mole fraction of water = 13.9 /(13.9 + 0.29) = 13.9/ 14.19 = 0.98

Therefore, the partial pressure of the solution = 0.98 × 23.8= 23.31mmHg

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