When two volatile liquids mix to form an ideal solution, both components appear

Question

When two volatile liquids mix to form an ideal solution, both components appear in the vapor phase over the solution. Each liquid can be viewed as either the solute or the solvent, and each lowers the vapor pressure of the other.

When the solution behaves ideally, Raoult's law can be used to calculate the partial pressure of each of the components over the solution. This defines the composition of the gas phase in equilibrium with the solution.

At 305 K the vapor pressure of pure chloroform (CHCl3) is 0.344 atm and the vapor pressure of pure carbon tetrachloride (CCl4) is 0.195 atm. Equal chemical amounts (equal numbers of moles) of chloroform and carbon tetrachloride are mixed and form an ideal solution. Compute the mole fraction of carbon tetrachloride in the vapor in equilibrium with this solution.

Explanation / Answer

Apply raoults law

x1*P°1 = y1*PT

x2*P°2 = y2*PT

and we know that y1+y2 = 1

so...

initially

P°1 = 0.344 and P°2 = 0.195

initially, x1 = x2 = 0.5 since equimolar and P = 1 atm (assume)

so

0.5*0.344 = y1*1

0.5*0.195 = (1-y1)*1

y1 = 0.172 for CHCl3 in vapor pressure

so

y2 = 1-0.172 = 0.828 in CCL4

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