At 363 K, the vapour pressure of A is 21 kPa and that of B is 18 kPa. One mole of A and 2 moles of B are mixed. Assuming that this solution is ideal, the vapour pressure of the mixture is ________ kPa. (Round off to the Nearest Integer).

We are given a binary ideal solution of A and B at 363 K, with vapour pressures of pure components $$P_A^0 = 21$$ kPa and $$P_B^0 = 18$$ kPa. The mixture contains 1 mole of A and 2 moles of B.

The mole fractions are: $$x_A = \frac{1}{1+2} = \frac{1}{3}$$ and $$x_B = \frac{2}{1+2} = \frac{2}{3}$$.

By Raoult's law for an ideal solution, the total vapour pressure is: $$P_{total} = x_A P_A^0 + x_B P_B^0 = \frac{1}{3} \times 21 + \frac{2}{3} \times 18 = 7 + 12 = 19$$ kPa.

The vapour pressure of the mixture is $$19$$ kPa.