Question
$$3\,moles$$ of $$P$$ and $$2\,moles$$ of $$Q$$ are mixed, what will be their total vapour pressure in the solution if their partial vapour pressures are $$80$$ and $$60\,torr$$ respectively ?
A.
80$$\,torr$$
B.
140$$\,torr$$
C.
72$$\,torr$$
D.
70$$\,torr$$
Answer :
72$$\,torr$$
Solution :
$$\eqalign{
& {\text{Mole fraction of}}\,\,P = \frac{3}{{3 + 2}} = 0.6 \cr
& {\text{Mole fraction of}}\,\,Q = \frac{2}{{3 + 2}} = 0.4 \cr
& {P_{{\text{total}}}} = {p_P} + {p_Q} = p_P^ \circ {x_P} + p_Q^ \circ {x_Q} \cr
& \,\,\,\,\,\,\,\,\,\,\,\, = 80 \times 0.6 + 60 \times 0.4 = 72\,torr \cr} $$