Raoult's law

When Raoult's law the requirement imposed by François Marie Raoult law is called, which describes the vapor pressure of ideal mixtures.

Lowering of vapor pressure

In the mixture of a solution of the solution effect occurs. This means that the saturated vapor pressure of the solvent to a solution having a flat surface ( curvature effect) will be lower than that of the pure solvent:

Here are the individual symbols for the following sizes:

• - Partial pressure or vapor pressure of the solvent above the solution
• - Mole fraction or relative lowering of vapor pressure of the solvent   - Number of moles of solute B and the solvent A

The reduction in vapor pressure is so colligatively, i.e. dependent only on the fuel quantity or of particles of the solute and not on the chemical properties.

Analogously, for the solute:

With

• - Mole fraction or relative lowering of vapor pressure of the solute ( for dilute solutions tend to be ).

Coupling equations (see figure):

And

With

• - The total vapor pressure above the solution.

Note:

• The partial pressure of a component to the solution is always less than that of the component in its pure form, even if the total vapor pressure over the solution is greater than ( in the Figure: even ).
• And are independently of one another. It can therefore - unlike in the picture - also happen that (or in the Fig: ).

Boiling point elevation

The above solution effect is responsible for the elevation of boiling point, i.e., the boiling point of the solution is higher than that of the pure solvent:

For the boiling point increase is due to the Clausius -Clapeyron equation, the following relationship:

The proportionality constant (also called ebullioscopic constant ) depends - apart from the general gas constant R - only from the following properties of the solvent A from:

• T - temperature in K
• - Molar heat of vaporization, ie heat of vaporization per mole.

Freezing point depression

The solution effect is also responsible for the freezing point depression that is, the melting or. Freezing point of the solution is lower than that of the pure solvent:

For the freezing point depression arises with the Clausius -Clapeyron equation, the following relationship:

The proportionality constant (also called cryoscopic constant ) depends - apart from the general gas constant R - only from the following properties of the solvent A from:

• T - temperature in K
• - Molar heat of solidification, ie heat of solidification per mole; corresponds to the molar heat of fusion, but with a negative sign.

Determination of the molar mass

Both from the boiling point elevation (ebullioscopy ) as well as from the freezing point depression ( cryoscopic ), the molar concentration, and thus the molar mass of the solute in [g / mol] are determined:

This works, because - as said - the reduction in vapor pressure does not depend on the nature of the solute, only on its concentration in the solution (if the solution is approximately ideal).