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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: gidget on March 02, 2009, 10:45:51 AM

Title: Mass-Balance Expressions
Post by: gidget on March 02, 2009, 10:45:51 AM

Write the mass-balance expression and the Charge-Balance equations for a solution that is:
a) 0.10M in Na₂HPO₄
b) 0.25M in NaF and saturated with CaF₂
c) Saturated with MgCO₃

* I am totally lost on these since the prof hadn't gone over any examples in the lecture. Any help would be GREATLY appreciated! Thank you!

Title: Re: Mass-Balance Expressions
Post by: Borek on March 02, 2009, 01:41:19 PM

here goes an example:

You have a 0.1M solution of phosphoric acid.

Mass balance (sum of all forms of acid present equals its initial concentration):

[H₃PO₄] + [H₂PO₄^-] + [HPO₄^2-] + [PO₄^3-] = 0.1

Charge balance (solution is neutral):

[H₂PO₄^-] + 2[HPO₄^2-] + 3[PO₄^3-] + [OH^-] = [H⁺]

If you have solids present you may need to express mass balance in terms of not concentrations, but numbers of moles.