At 20°C the vapor pressure of benzene (C6H6) is 75 torr, and that of toluene (C7H8) is 22 torr. Assume that benzene and toluene form an ideal solution. What is the composition in mole fractions of a solution that has a vapor pressure of 40. torr at 20°C?
1 Answer
Explanation:
The idea here is that the vapor pressures of benzene and toluene will contribute to the total vapor pressure of the solution proportionally to their respective mole fraction - this is known as Raoult's Law.
Mathematically, you can express this by the following equation
P_"sol" = chi_"benzene" * P_"benzene"^@ + chi_"toluene" * P_"toluene"^@Psol=χbenzene⋅P∘benzene+χtoluene⋅P∘toluene , where
Now, mole fraction is defined as the ratio between the number of moles of a component of a solution and the total number of moles present in the solution.
Since you only have two components to this solution, benzene and toluene, you can say that
chi_"benzene" + chi_"toluene" = 1χbenzene+χtoluene=1
Let's say that the mole fraction of benzene is
x = 1 -yx=1−y
and
P_"total" = x * P_"benzne"^@ + (1-x) * P_"toluene"^@Ptotal=x⋅P∘benzne+(1−x)⋅P∘toluene
40. color(red)(cancel(color(black)("torr"))) = x * 75color(red)(cancel(color(black)("torr"))) + (1-x) * 22color(red)(cancel(color(black)("torr")))
40. = 75x + 22 - 22x
40 - 22 = 53x implies x = 18/53 = 0.3396
This means that you have
y = 1 - 0.3396 = 0.6604#
The mole fractions of benzene and toluene in the solution will thus be - rounded to two sig figs
chi_"benzene" = color(green)(0.34)" " and" "chi_"toluene" = color(green)(0.66)
