1. Use Raoult’s law for an ideal solution

For an ideal solution, total vapour pressure is:

$P_{\text{total}} = x_A P_A^0 + x_B P_B^0$

where:

• $x_A, x_B$ are mole fractions in the liquid phase
• $P_A^0 = 21\,\text{kPa}$
• $P_B^0 = 18\,\text{kPa}$

2. Calculate mole fractions

Given:

• moles of $A = 1$
• moles of $B = 2$

Total moles:

$n_{\text{total}} = 1 + 2 = 3$

So,

$x_A = \frac{1}{3}, \qquad x_B = \frac{2}{3}$

3. Substitute into Raoult’s law

$P_{\text{total}} = \frac{1}{3}(21) + \frac{2}{3}(18)$

$P_{\text{total}} = 7 + 12 = 19\,\text{kPa}$

4. Final answer

Rounded to the nearest integer:

$\boxed{19}$

5. Comparison with stored correct answer

Stored correct answer = $19$

My derived answer matches the stored correct answer.