A soultion of benzene C 6 H 6 and toulene C 7 H 8 is 25% benzene by mass. At 25C

Question

A soultion of benzene C6H6 and toulene C7H8 is 25% benzene by mass. At 25C the vapor pressures of pure benzene and pure toulene are 94.2 torr and 28.4 torr respectively. Assuming ideal behavior, calculate the following:

A.) Vapor Pressure of each solution components in the mixture

B.) Total pressure above the solution

C.) The composition of the vapor in mass percent

Why is the composition of the vapor different from the composition of the solution?

The final answers are

26.6 Torr for the benzene

20.4 Torr for the toulene

47.0 Torr for the total solution

My own calculations are slightly different:

21.4 torr for the benzene

21.9 torr for the toulene

43.4 torr for the total

I can't figure out what i did wrong. I used the PA=XAPoA and PB=XBPoB

Now, from my understanding, the something is the percent mass of something, like this problem for example, the Benzene is 25 grams for every 100 grams of toulene. Therefore, it would be 25gbenzene/100gtoulene. Am I wrong?

Explanation / Answer

Yes. You are wrong. mass percent means mass of solute/ mass of solution.

Thus 25% benzene by mass means 25 g of benzene in 100 go solution, which makes 75 go toluene .

Thus moles of benzene = 25/78 =0.3205

and moles of toluene = 75/92 =0.8152

Thus mole fraction of benzene = 0.3205 /(0.3205+ 0.8152)

=0.2822

and mole fraction of toluene = 0.7178

Vapor pressure of benzene over solution = mole fraction x vapor pprssure of pure benzene

= 0.2822 x 94.2 torr

= 26.58 torr

Vapor pressure of toluene = 0.7178 x 28.4

= 20.38 torr

Total vapor pressure of solution = P9benzene ) + P(tolune)

= 26.58 + 20.38

= 46.96

= 47 torr

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.