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### Topic: Calculate the percent by mass? using molality  (Read 21972 times)

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#### Craftylikeafly

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##### Calculate the percent by mass? using molality
« on: January 30, 2010, 06:33:54 PM »
A solution of caffeine (C8H10N4O2) in chloroform (CHCl3) as a solvent has a concentration of 7.10*10^-2m.

I know that i have to convert the molality to grams somehow to get the mass of the solute but i am stuck, please help with detail, i also know the m=mols of solute/kg of solvent, caffeine is the solute and the chloroform is the solvent and the mass %= mols of solute/total mass*100

#### Black Mage

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##### Re: Calculate the percent by mass? using molality
« Reply #1 on: January 31, 2010, 12:04:07 AM »
Have you tried assuming a given amount of solvent?

EDIT: I would double check your % mass formula.
« Last Edit: January 31, 2010, 12:18:30 AM by Black Mage »
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#### Craftylikeafly

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##### Re: Calculate the percent by mass? using molality
« Reply #2 on: January 31, 2010, 09:10:54 AM »
I still dont understand

#### Black Mage

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##### Re: Calculate the percent by mass? using molality
« Reply #3 on: January 31, 2010, 01:25:05 PM »
Assume 1kg of chloroform is the solvent.

Molality = (mol of solute) / (kg of solution)

So now you should have mol of solute. Use stoichiometry from there to get the mass of caffeine in this solution.

% Mass = (Mass of solute) / (Mass of Solute + Mass of Solution)

Remember we assumed a mass of solution in the first step.