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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: mich3ll3 on March 08, 2012, 09:13:18 PM

Title: please help me on a chemistry problem!
Post by: mich3ll3 on March 08, 2012, 09:13:18 PM

MASTERINGCHEM ?: An aqueous solution contains 15.0% NaCl by mass.?
What mass of water (in grams) is contained in 3.0 L of the vapor above this solution at 55 degrees Celsius? The vapor pressure of pure water at 55 degree Celsius is 118 torr . (Assume a van't Hoff factor of 1.9 for NaCl.)

The answer should be in grams.

I've been trying to figure it out for a while now. Please show me the steps. I have some parts of it such as the mole fraction ect but i don't know what to do to come up with the right answer! HELPPPP!

Title: Re: please help me on a chemistry problem!
Post by: Borek on March 09, 2012, 03:33:54 AM

Show what you did so far.

Title: Re: please help me on a chemistry problem!
Post by: mich3ll3 on March 09, 2012, 10:17:40 AM

15.0%NaCl =  15.0 g /100 g solution
15g/58.5 (g/mol) = 0.256 mole
mole corr = 0.256 x 1.9 = 0.487 mole

100-15 = 85
mol H2O =  85g H2O / 18 g = 4.72 mole
mol fraction NaCl = 0.487/4.72 = 0.103

vp lowering = 118 x 0.130 = 15.34

Title: Re: please help me on a chemistry problem!
Post by: Borek on March 09, 2012, 12:33:37 PM

Quote from: mich3ll3 on March 09, 2012, 10:17:40 AM

mol fraction NaCl = 0.487/4.72 = 0.103

Close, but that's not how you calculate molar fraction.

Title: Re: please help me on a chemistry problem!
Post by: mich3ll3 on March 11, 2012, 02:38:24 PM

ohhh thank yoU! soo its

((.487)/(4.72+.487)) x 118 = 11.036 = vp lowering

now what ?

Title: Re: please help me on a chemistry problem!
Post by: Borek on March 11, 2012, 02:46:21 PM

Calculate P and use ideal gas equation.