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Mass/Charge Balance, Am I doing this correctly?

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DeltaSubz:
Hey all, first post here  :D
Anyways, it seems there will be some mass/charge balance questions on my upcoming quant midterm and want to make sure i get this right. Im pretty sure i got the hang of mass balance, but not so much charge balance. An example problem is as follows, and my answers are in red...

A solution is prepared using 0.01 mol of Na2HPO4, 0.02 mol of Ca(H2PO4)2, and 0.02 mol of Mg3(PO4)2. In a total volume of  0.50 L.

a.   Write all mass balance equations for this solution
[Na+] = 0.04 M
[Ca2+] = 0.04 M
[Mg2+] = (3*0.02)/0.5 = 0.12 M
[H3PO4] + [H2PO4-] + [HPO42-] + [PO43-] = ( 0.01 + (2*0.02) + (2*0.02))/0.5 = 0.18 M

b.   Write the charge balance equation for this solution. Where appropriate, replace general concentration terms with actual concentrations.
[H3O+] + (0.36 M) = [H2PO4-] + 2[HPO42-] +3[PO43-] + [OH-]

Can anyone knowledgeable in this area verify whether this is right/wrong?

Borek:
I just skimmed over your numbers and nothing sticks out as obviously incorrect, but there is one thing that I would do differently:


--- Quote from: DeltaSubz on March 21, 2019, 05:24:25 AM ---[H3O+] + (0.36 M) = [H2PO4-] + 2[HPO42-] +3[PO43-] + [OH-]
--- End quote ---

Instead of writing 0.036 M I would first list all ions with coefficients. Makes it easier to check what you are doing.

AWK:
Charge balance and mass balance will be correct assuming that all compounds are soluble in water and do not react chemically between each other.
But this is not  the case. The solubility of Mg3(PO4)2 is of the order of 10-6 M.

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