March 28, 2024, 07:03:22 PM
Forum Rules: Read This Before Posting


Topic: Calculate the percent by mass? using molality  (Read 23185 times)

0 Members and 1 Guest are viewing this topic.

Offline Craftylikeafly

  • Regular Member
  • ***
  • Posts: 10
  • Mole Snacks: +0/-0
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


 

Offline Black Mage

  • Regular Member
  • ***
  • Posts: 21
  • Mole Snacks: +1/-0
  • Gender: Male
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 »
"Let the future tell the truth and evaluate each one according to his work and accomplishments. The present is theirs; the future, for which I really worked, is mine." - Nikola Tesla

Offline Craftylikeafly

  • Regular Member
  • ***
  • Posts: 10
  • Mole Snacks: +0/-0
Re: Calculate the percent by mass? using molality
« Reply #2 on: January 31, 2010, 09:10:54 AM »
I still dont understand  :(

Offline Black Mage

  • Regular Member
  • ***
  • Posts: 21
  • Mole Snacks: +1/-0
  • Gender: Male
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.

Hopefully this is more helpful. :)
"Let the future tell the truth and evaluate each one according to his work and accomplishments. The present is theirs; the future, for which I really worked, is mine." - Nikola Tesla

Sponsored Links