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Finding Molecular Weight

  1. Nov 3, 2013 #1
    A compound contains boron (40.3%), nitrogen (52.2%), and hydrogen (7.5%) - by mass. If 6.32g of this compound if dissolved in 100g of benzene, the solution freezes at 1.30 celcius. If the cryoscopic constant is 5.12 and freezing pt of benzene is 5.48, what is the molecular weight of compound?

    I first rearranged the freezing pt depression equation to solve for the molarity of the solution (in mol of solute per kg of solvent), then multiplied this by the mass of of the solvent to obtain number of moles of solute. Being given the mass (6.32g) of the compound, I divided this mass by the number of moles calculated to obtain the molecular weight of the compound. Is this the correct approach?
    I can calculate the actual molecular formula by using the % by mass and the molecular weight computed.
  2. jcsd
  3. Nov 3, 2013 #2


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    Staff: Mentor

    Your description of what you did sounds OK - just remember you have calculated molality, not molarity.

    Unless you have quoted only part of the problem, you are not asked to find the molecular formula, so you can safely ignore the composition information.
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