chads quiz question

[dentcal]

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i was working out one of chads quizes but i came across a question and didn't understand how he got mols of H2O to plug it into the mol fraction. here's the question:

If the vapor pressure of pure water at 20°C is 18 torr, what is the vapor pressure of water above a solution of 5.5m CH3OH?

the answer is 16.4 and the quiz is from video 4.2 Colligative Properties question number 480.

 

[sacapuntas]

i was working out one of chads quizes but i came across a question and didn't understand how he got mols of H2O to plug it into the mol fraction. here's the question:

If the vapor pressure of pure water at 20°C is 18 torr, what is the vapor pressure of water above a solution of 5.5m CH3OH?

the answer is 16.4 and the quiz is from video 4.2 Colligative Properties question number 480.

5.5m CH3OH means 5.5 moles CH3OH for 1 kg of water (lower case m means molality). You can figure how many moles of water are in a kg by dividing 1000 g water / 18 g/mol = 55.55 moles of water. So to find mole fraction,

(55.55 moles water) / (55.55 moles water + 5.5 moles CH3OH) = about 0.91

0.91 is the mole fraction, multiply that by 18 torr to get the 16.4 torr.

 

[Tangymoose]

I am going to take a random shot in the dark with this because I'm not familiar with his quizzes but I believe its because the problem says "5.5m CH3OH" which is molality.

Molality is moles of solute per kg of solvent (water) so I'm guessing the problem has 1kg of water & he converted that to moles of water

 

[dentcal]

thanks for the help guys i understand it now!!!!😀