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Homework Help: Determining molecular formula of a hydrate? (HELP!)

  1. Nov 15, 2012 #1
    1. The problem statement, all variables and given/known data
    mass of hydrated comlex (g) - 6.32
    mass of anhydrous complex (g) - 3.54
    mass of water (g) - 2.78

    MolarMass MgSO4 = 120.38g/mol
    MolarMass H2O = 18.02g/mol

    Based on those results, determine the molecular formula of the hydrate.

    2. Relevant equations

    3. The attempt at a solution
    nMgSO4 = 3.54/120.38 ---> 0.0294068782189733
    nH2O = 2.78/18.02 ----> 0.1542730299667037

    then to find the multiplier I must divide by the lowest mol;

    0.0294068782189733/0.0294068782189733 = 1
    0.1542730299667037/0.0294068782189733 = 5.246154617907276 / 5.25

    therefore, for every MgSO4, there is 5.25H2O

    Molecular formula: Mg4SO16*21H2O (since our teacher asks for integers only)

    is this correct, or is it incorrect, help please!!!
  2. jcsd
  3. Nov 16, 2012 #2


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    Homework Helper
    Education Advisor
    Gold Member

    The mole ratio you find between the anhydrous salt and the water is good, but maybe as for ratio of integers, you need larger numbers, so as to have a whole number for MgSO4 larger than 1. You will no longer have a "5" for the corresponding H2O part. Multiply, "0.25" by what to get a whole number? Then what do you do for the MgSO4 part?

    I'm thinking more like,
    5.25*4, H20, and 1*4, MgSO4.
    Formula unit would be (MgSO4)4*21H2O
  4. Nov 16, 2012 #3


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    Staff: Mentor

    Note that hydrates often don't have nicely defined formulas and amount of crystalline water can depend on the humidity and the sample history (what was the temperature/humidity it was kept in lately). Often the composition is different on the surface and in the bulk. Add to that fact that measurements are always inaccurate, so you can't expect the ratio to be exactly an integer number.

    I would go for 5H2O (rounding down) with a comment explaining why the result is what it is. Whether it will be accepted and praised depends on the teacher.
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