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College Chemistry

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a mixture contains two crystalline solid X and Y. Decide if pure X can be obtained by a proper recrystallization using solvent A. Include the exact volumes of solvent used and the amount of pure X obtained.

Solubility of X in boiling A :200mg/ml
solubility of X at room temp.: 20mg/ml
solubility of Y at boiling A : 200mg/ml
solubility of Y at room temp.: 20mg/ml
amount of X in mixture: 400mg
amount of Y in mixture: 30mg

  • College Chemistry -

    Suppose you take 2 mL of boiling A.
    X = 2 mL x 200 mg/mL = 400 mg so all of X will dissolve.

    Y = 2 mL x 200 mg/mL = 400 mg so all of Y will dissolve.

    Now cool the solution to room temperature. How much X and Y will recrystallize?
    2 mL x 20 mg/mL = 40 mg X will stay in solution; 400 - 40 = 360 mg X will crystallize.

    For Y,
    2 mL x 20 mg/mL = 40 mg Y will stay in solution; you had only 30 so none will crystallize.

  • College Chemistry -

    I'm guessing that 2mL of boiling A came from (400mg)(1mL/200mg)= 2mL

    So we don't have worry about the Y at boiling A? which will be 0.15mL if we do the calculation.

  • College Chemistry -

    You add enough boiling A to dissolve all of X (of course it will dissolver all of Y, too). Using 0.15 mL of boiling A will dissolve all of Y and some of X. In fact I suppose you could get pure X this way, too, although it isn't the usual way we think of recrystallization.
    200 mg/mL x 0.15 mL = 30 mg X will dissolve leaving 400-30 = 370 mg X behind, BUT
    1. 0.15 mL is a small volume to be working with, and you would have trouble controlling the volume and losing volume as it sticks to the walls of the vessel,
    2. I would want to add just a little more than 0.15 because this JUST barely gets all of Y in solution.
    3. I doubt this is the way your prof expects you to answer.

  • College Chemistry -

    I was also thinking so we can't add the two volume of A together? So like for X in boiling A there is 2mL and in Y there is 0.15ml. So all together solvent A would be 2.15mL.

    all of Y would still be gone but for X instead of 360mg i would get 357mg

  • College Chemistry -

    In general, we assume that we need only one volume. In this case, however, an extra 0.15 mL couldn't hurt anything. As you point, however, it affects the yield.

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