Question

A solution of methylene chloride (CH2C2) and ethanol CHCH2H) is boiling at 70.8 °C. A sample of the vapor above the solution is cooled until it condenses. This condensed sample is analyzed, and turns out to be 50.90 methylene chloride by mass. Calculate the percent by mass of methylene chloride in the boiling solution. Here's some data you may need vapor pressure at 70.8 °C normal boiling point density 1.3 2009. torn methylene chloride mL 0.79 g mL 9568. torr 78. °C0 ethanol Be sure your answer has 2 significant digits Note for advanced students: you may assume the solution and vapor above it are ideal.

Answer 1

We can assume that the boiling is happening at the external pressure.

From the concept of the boiling we can say that the boiling happens only when the external pressure is equal to the pressure applied by the gases.
From Daltons law we know pressure is given by
Summation (PtYi)
Here Pt is the total external pressure.
Pt= 1 atm = 760 torr
Yi= 0.5

Also from the Roults law we know that
Pi*Xi = Pt*Yi
Here Pi is vapor pressure of component i

In our question
Pi=2009 torr
Xi = Pt*Yi/Pi
Xi= ((770*0.5)/2008)

Xi = 0.189

Here Xi is the mole fraction of the methylene chloride in the boiling solution.

The answe is subject to the assumptions of the external pressure equal to 760 torr.

Hope it helps .
Thank you