Question

What is Raoult's Law of vapor-pressure? Could someone please explain with diagrams?

Answer 1

Raoult's law just says that the vapor pressure `P_A^"*"` above a pure liquid will decrease to `P_A < P_A^"*"` when solvent is added into it.

For ideal mixtures (no change in intermolecular forces after mixing), it is based on the mole fraction `chi_(A(l))` of solvent in the solution phase:

`P_A = chi_(A(l))P_A^"*"`

where `A` is the solvent.

Since `0 < chi_(A(l)) < 1`, it follows that the vapor pressure of the solvent must decrease. It starts out as `P_A = P_A^"*"`, and then as `chi_(A(l))` decreases, `P_A` decreases.

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The solute blocks the solvent from vaporizing, so it is harder to boil, and thus the vapor pressure is lower than desired; it is harder to reach the atmospheric pressure, so the boiling point is also higher.