Question

In the given pressure vs composition phase diagram (At constant T), the point at which both liquid and vapour coexist is

• A. Both $O$ and $Z$
• B. Both $Y$ and $M$
• C. Only at $N$
• D. Both $O$ and $Y$

Answer 1

The correct option is B Both $Y$ and $M$

The straight line in pressure composition at constant temperature is given by equation,
$P_T=(P_A^0-P_B^0)X_A+P_B^0$

The curvy line in the pressure composition diagram is given by plot of $P_{T}vs{Y}_A$

Let, initially the pressure over the solution is very high so that no vapour exist above the liquid.
On decreasing the pressure gradually, a point (bubble point) comes when we cross the bubble-point curve where first bubble
of vapour starts forming. It is the vapour-liquid equilibrium region.
The area between the two curves is vapour-liquid equilibrium region. The vapours cannot exist above the bubble point curve and liquid can’t exist below the dew point curve.
On further decreasing the pressure, a point (dew point) comes when we cross the dew-point curve where almost all the liquid has evaporated into vapour i.e. only the last drop of liquid (dew) remains. Beyond this point, no liquid exist in the system.

Only point $Y$ and $M$ are in the region between two curves where vapour and liquid are in equilibrium. Hence, at point $Y$ and $M$ both liquid and vapour phase will exist.