For the same binary solution (P°₁ = 450 mm Hg, P°₂ = 700 mm Hg, total P = 600 mm Hg, $x_1 = 0.4$, $x_2 = 0.6$), the mole fraction of B (component 2) in the **vapour phase** is:

$P_2 = x_2 P_2^0 = 0.6 \times 700 = 420$ mm Hg. By Dalton's law $y_2 = P_2/P = 420/600 = 0.70$. (Note: vapour is **enriched** in the more volatile component B — its mole fraction rises from 0.6 in liquid to 0.7 in vapour.)