Themes > Science > Chemistry > General Chemistry > Solution and Solubility > Solubility of Solutes and Aqueous Solutions > Solution Index > Colligative properties > Vapor pressure lowering


In general, solutions have a lower vapor pressure than does the pure solvent. This vapor pressure lowering is an example of a colligative property, one which depends only on the number of moles of the solute. not the identity of that solute. The boiling point elevation of solutions is really an effect of the vapor pressure lowering: since the boiling point of a solution is the point at which its vapor pressure is the same as the external pressure, if you lower the vapor pressure you'll raise the boiling point. Freezing point depression is also a result of vapor pressure lowering. Can you figure out why? (Hint: draw a phase diagram of the solution as compared to the pure solvent.)

The equation governing the change in the vapor pressure when you use a nonvolatile liquid or solid is just a special case of Raoult's Law, Ptot = PaXa + PbXb. Since the second component has a negligible vapor pressure (nonvolatile), Raoult's law reduces to

P = P0X
where P is the vapor pressure of the mixture, P0 is the vapor pressure of the pure solvent and X is the mole fraction of the solvent.

When working with ionic solutes in water and other polar solvents, one must be careful to take into consideration the fact that the ion concentration is higher than the concentration of the solute. For example, table salt, NaCl dissolves in water to form ions

NaCl(s) -> Na+(aq) + Cl-(aq)
Each mole of NaCl makes two moles of ions, so if we have a 1 molal solution of NaCl in water, the ion concentration is 2 molal.

Example: At 35oC, the vapor pressure of water is 43.4 mmHg. What is the vapor pressure of a 1.00 molal solution of NaCl?

Solution: First, we need to figure out the mole fraction of water in the solution, then apply the above equation for the vapor pressure lowering. 1.00 molal means 1 mole solute in 1 kg water. 1 kg water = 1000 g water. Water has a molecular weight of 18.01 g/mole, so 1000 kg water/18.01 g/mole = 55.5 moles of water.

We have to be careful about the number of moles of solute. NaCl ionizes into Na+ and Cl- ions in water, so we have 2 moles of ions in the solution, not 1.

We have 2 moles of ions and 55.5 moles of water in 1 L of solution, so we have a total of 57.5 moles of solution. The mole fraction of water is thus

Xwater = 55.5 moles water/57.5 moles total = 0.965
Now just use the equation above.
P = P0X
P = 43.4 mmHg * 0.965
P = 41.9 mmHg


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