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Math Help - mass of solid, chemistry calculation.

  1. #1
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    mass of solid, chemistry calculation.

    5 kg of anhydrous  CuSO_{4} are mixed with 20 kg of water. The mixture is then kept at  20^{o}C until the maximum possible amount of   CuSO_{4} has dissolved. Any undissolved  CuSO_{4} is hydrated to form solid

      CuSO_{4}.5H_{2}O . Calculate the mass of undissolved solid.

    Data: Molecular Masses:

     CuSO_{4}= 159.6

     CuSO_{4}.5H_{2}O = 249.7

    Solubility at   20^{o}C : 20.7kg  CuSO_{4} per 100 kg water.

    I am bit confused on how to do this, as I am no sure what I am suppose to be working out? Is it the amount of copper sulphate in the hydrated solid?

    Any help appreciated.

    Thank you.
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  2. #2
    MHF Contributor Unknown008's Avatar
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    From the solubility you were given, find the mass of CuSO4 that dissolves in the 20 kg of water.

    Then, work out the mass of undissolved CuSO4. They are converted to the hydrated crystals.

    Write the equation!

    CuSO_4 + 5H_2O \rightarrow CuSO_4\cdot 5H_2O

    You have the mass of CuSO4 which crystallises. Find the number of moles of it. Hence, deduce the number of moles of hydrated crystals formed, then its mass.
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  3. #3
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    Thank you;


    I think I get what your saying;

    so  \frac{20.7}{100} = 0.207 kg that is the solubility for 1kg of water, so multiply by 20 to get the solubility for 20kg of water

    0.207 x 20 = 4.14 kg

    5 - 4.14 = 0.86kg this is the mass of undissolved copper sulphate that turns into the crystal right?

    I am bit confused about the next step, as I have already found the mass of the undissolved solid? (0.86kg)

    but to work out the moles of CuSO4 =  \frac{5}{159.6}  = 0.031 moles kg^{-1}

    not sure where to go from here?


    Thanks a lot for your help.
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  4. #4
    MHF Contributor Unknown008's Avatar
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    5 kg is the total mass of CuSO4. You are looking for the undissolved mass of CuSO4.

    4.41 kg dissolved and 0.86 kg is undissolved. Hence, you want to find (0.84 x 1000)/159.6 mol of undissolved CuSO4.

    This much is converted to crystals. Can you continue?
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  5. #5
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    Okay, so that give me 5.388 mol , so the equation is in a 1:1 ratio, so the mass of the crystal is 5.388 x 249.7 = 1345.5 kg , which is not the right answer?
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  6. #6
    MHF Contributor Unknown008's Avatar
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    Not quite. I multipled 0.86 (I did a typo earlier, sorry for that...) by 1000 to get the mass in grams.

    And hence, you'll get 1345.5 g and not kg.

    Don't you think that it's a little weird to get more than a tonne starting with 25 kg of reagents?
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  7. #7
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    so the correct answer is 1.3445 kg ?
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  8. #8
    MHF Contributor Unknown008's Avatar
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    I don't have a calculator with me right now, but I will trust you with the calculations.

    It seems good to me.
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