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Homework Help: Molarity given percent solution and density.

  1. Sep 6, 2009 #1
    1. The problem statement, all variables and given/known data
    Lactose (C12H22O11; MM=342) is a sugar found in mamalian milk. The density of a 12.0% solution is approximately 1.06 g/mL. What is the molarity, molality, and mole fraction of the solution?

    2. Relevant equations
    M=g/L, m=mol/kg, molefraction=mol/mol??

    3. The attempt at a solution
    I think I can get this one if I can just determine the molarity. I definitely did something wrong.

    1.06 / 1000 = .00106; .00106/342 = 3.099x10^-6M
    I figured since I know g/mL as well as mol/g, I can divide to get mol/mL, then convert to get L. Apparently this is not the case?
  2. jcsd
  3. Sep 6, 2009 #2
    You're on the right track, just a few things you need to pay attention to:

    1.06 grams of solution contains how many grams of lactose?

    Also, show your units at all steps of the calculation to make sure you're converting correctly.
    Last edited: Sep 6, 2009
  4. Sep 7, 2009 #3


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    Staff: Mentor

    Last edited: Sep 7, 2009
  5. Sep 10, 2009 #4
    Hmm, then let me try this again. I just want to find the moles of Lactose for now, then I think I got the rest.

    1.06 g/mL * 1mol/342g * 12% lactose = 3.719x10^-4

    Does that look right?

    Thanks for the help btw.
  6. Sep 10, 2009 #5
    That looks good, but remember to keep track of the units.
  7. Sep 11, 2009 #6


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    Staff: Mentor

    Moles of lactose in what?

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