Question 1

What does Raoult's law state?

Accepted Answer

For an ideal solution of volatile components, the partial vapour pressure of each component equals its liquid-phase mole fraction multiplied by the vapour pressure of that pure component at the same temperature. The total equilibrium pressure is the sum of these partial pressures.

Question 2

Why is the vapour phase richer in the more volatile component?

Accepted Answer

For an interior binary composition, the more volatile component is enriched in the vapour relative to the liquid because its pure-component vapour pressure is larger. At equal liquid mole fractions, its partial pressure is also larger, since p_i = x_i p_i^0 and p_i^0 is larger for the more volatile component.

Question 3

What happens when a non-volatile solute is added?

Accepted Answer

The non-volatile solute does not contribute to the equilibrium vapour pressure, so p = x_solvent p_solvent^0. Its presence lowers the pressure below that of the pure solvent, and the relative lowering is (p_solvent^0 - p)/p_solvent^0 = x_solute.

Question 4

How are positive and negative deviations from Raoult's law shown?

Accepted Answer

A positive deviation gives a total vapour pressure higher than x_A p_A^0 + x_B p_B^0. A negative deviation gives a total pressure lower than that ideal Raoult-law value.

Question 5

What is the difference between liquid and vapour composition?

Accepted Answer

Liquid composition is written using x_A and x_B, where x_A = n_A/(n_A+n_B). Vapour composition uses y_A = p_A/P and y_B = p_B/P. Multiplying either mole fraction by 100 gives its mole per cent.

Raoult's Law at the Liquid Surface: Who Escapes?

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