= Ideal binary vapor composition
{title2=$y_A=\frac{x_Ap_A^*}{x_Ap_A^*+(1-x_A)p_B^*}$}
For the stipulated A/B <ideal solution>, put $x=x_A$. At the fixed <temperature> $300\,\mathrm K$, <Raoult's law> reduces to
$$
P=(20+60x)\,\mathrm{kPa},\qquad
y_A=\frac{80x}{20+60x}=\frac{4x}{1+3x}.
$$
The <relative volatility> is $\alpha=p_A^*/p_B^*=4$. For $0<x<1$,
$$
y_A-x=\frac{3x(1-x)}{1+3x}>0.
$$
Hence every interior <vapor-liquid equilibrium> pair in this chosen model has A-enriched <vapor>; neither pure endpoint is enriched. Conversely,
$$
x_A=\frac{y_A}{4-3y_A}.
$$
For example, $y_A=1/2$ corresponds to $x_A=1/5$, $p_A=p_B=16\,\mathrm{kPa}$ and $P=32\,\mathrm{kPa}$. Equal <vapor> <mole fractions> therefore need not mean equal <liquid> <mole fractions>.
These are fixed-<temperature> predictions. During <boiling> at a specified external <pressure>, the <temperature> instead adjusts until $\sum_i x_i p_i^*(T)=P_{\rm external}$. The two stipulated <vapor pressures> specify only one <temperature>; they provide no <boiling>-<temperature> curve.
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