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Homework Help: Phosphorus in a solution of sodium phosphate

  1. Nov 18, 2013 #1
    1. The problem statement, all variables and given/known data

    I have a solution containing both mono basic (480mg/ml) and dibasic (180mg/ml) sodium phosphate. How do I determine the total mass of phosphorus per ml of the entire solution?

    2. Relevant equations


    3. The attempt at a solution

    Monobasic sodium phosphate (MSP):
    (480mg/mL)/(119.98mg/mmol)=4 mmol/mL

    Phosphorus in MSP: (30.974 mg/mmol)(4 mmol/mL)=123.90 mg/mL

    Dibasic sodium phosphate (DSP)
    (180 mg/mL)/(141.96mg/mmol)=1.268 mmol/mL

    Phosphorus in DSP: (30.97 mg/mmol)(1.268 mmol/mL)=39.28 mg/mL

    Total elemental phosphorus = 123.90+39.28=163.18 mg/mL

    However, this answer is not correct. Apparently, the correct answer is 124.36 mg/mL
  2. jcsd
  3. Nov 18, 2013 #2


    User Avatar

    Staff: Mentor

    163 mg/mL looks OK to me.

    Are you sure about concentrations given? I have no solubility tables at hand, but 480 mg/mL looks high. Not necessarily impossible, just worth checking.
  4. Nov 19, 2013 #3
    It is dissolved in water and glycerin, which likely improves the solubility.

    I've found an alternative solution to the problem, but now I don't know how to justify one over the other. Can someone explain why the following answer is the correct answer versus the one above?

    Here is the alternate solution (using some of the calculations made above):

    [MSP]=4 mmol/ml (see above)
    [DSP]=1.268 mmol/ml (see above)
    Molar mass phosphate ion: 94.97 mg/mmol
    Molar mass MSP: 119.98 mg/mmol
    Molar mass DSP: 141.96 mg/mmol

    Phosphate ion concentration:
    [(94.97/119.98)x4 mmol/ml]+[(94.97/141/96)x1.268 mmol/ml]=3.1667+0.8483=4.015 mmol/ml

    Elemental phosphorus in phosphate ions:
    (30.974 mg/mmol)x(4.015 mmol/ml)=124.36 mg/ml
  5. Nov 19, 2013 #4


    User Avatar

    Staff: Mentor

    Solution that 4 mmol/mL in MSP is also 4 mmol/mL in phosphate (and sodium).

    Your previous answer was correct, now you are just juggling numbers.
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