Question

Consider the following partially labelled figure for an ideal binary solution of benzene and toluene and answer the following questions:
What should be the pressure above which a solution containing 60% (by moles) toluene boils at $25{}^{\circ }\text{C}$?

• A. $760\text{torr}$
• B. $40\text{torr}$
• C. $60\text{torr}$
• D. $130\text{torr}$

Answer 1

The correct option is C $60\text{torr}$

The easiest way to solve this is using the graph. Look at the point in the graph where line 1 has a mole fraction of 0.6 - same as line 2 at 0.4. If you move up to line 3, you see that the pressure is 60 mm Hg which is the answer.

You can also do this by using the formulae of an ideal solution:
$P^{0}T=30mmHg$ and $P^{0}B=100mmHg$
Solution will start boilling when the pressure above solution is equal to total vapour pressure of the solution
$P_S=P^{0}T\times 0.6+P^{0}B\times 0.4=30\times 0.6+100\times 0.4=58mmHg$
Hence pressure above solution is approximately 60 mmHg