Sunday, April 21, 2013

Calculating pH of a solution with two weak acids

The pH of a solution that has two monoprotic weak acids such as acetic acid and benzoic acid can be calculated using the following equation:

Where, Ca1 and Ca2 are the concentrations in Molarity for the two acids, and Ka1 and Ka2 are the respective acid ionization constants for the two acids.

For example, assuming  a solution with 0.01 M acteic acid (Ka=1.76E-5) and 0.001 M benzoic acid (Ka=6.30E-5) then by applying the above equations we get pH = 3.31.

Keeping in mind that the above equation is correct given that Ca1 and Ca2 are much greater than Ka1 and Ka2, and greater than the hydronium ion concentration.

A free online pH calculator can be used to find solution for such pH problems.

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