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Homework Help: Does this problem require knowing the density of water?

  1. May 7, 2013 #1
    1. The problem statement, all variables and given/known data

    What is the molality of an aqueous solution of a compound with a molar mass of 120. g/mol that has a density of 1.56 g/mL and a molarity of 5.78 M?

    (A) 6.67 m
    (B) 5.78 m
    (C) 120. m
    (D) 1.56 m
    (E) None of the Above

    2. Relevant equations
    Molality= moles solute/kilograms solvent

    3. The attempt at a solution

    I assumed a volume of 1 L of solution, leaving me with 5.78 moles solute.

    5.78 moles solute x 120 g/mole solute = 693.6 g solute

    693.6 g solute x 1 mL/ 1.56 g solute =444.615 mL (the space taken up by the solute)

    1000 ml(b/c I assumed 1 L solution)- 444.615= 555.385 ml (volume of solvent, water)

    I don't know where to go from here b/c I don't know what the density of water is at whatever temperature the solution is at.
  2. jcsd
  3. May 7, 2013 #2


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    Staff: Mentor

    I believe you are misreading the question. 1.56 g/mL is density of the solution, not of the solute. If so, what is mass of the 1L of the solution? How much of this mass is water?

    If 1.56 g/mL is intended to be density of the solute, there is not enough information to solve the problem.
  4. May 8, 2013 #3
    Thanks so much Borek.

    Yes, I was misreading the problem. I tried solving it again using 1.56 g/mL as the density of the entire solution and I got the right answer, which is a) 6.67 molality.

    I gotta start reading these problems more carefully. :approve:
  5. May 8, 2013 #4


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    Staff: Mentor

    I am not surprised you misread the problem, it is lousy worded and confusing. For me it was obvious as I have seen zillions of such questions.
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