[born2dive00]

Ok I know that the Henderson-Hasselbach can determine the pH with a simple mono protic acid to give the pH

but
1. what is the formula to determine the pH of a of triprotic acid
2. what is the formula for say 2 or 3 triprotic acids say phosphoric acid, citric acid,
3. what is the formula for  tri protic acids + a mono protic phosphoric acid, citric acid, benzoic acid?
4. what is the formula for these buffered with sodium or potassium hydroxide?

The mol (concentration) of the
NaOH is 5.00 /L
Benzoic acid is 1.19/ L
Phosphoric acid is 3.71 / L
KOH is 2.29 / L
Citric acid 15.5 /L

Can anyone help me set up the formula needed?
Thank you
Brad

[Babcock_Hall]

You must show us your work before we can help you.  However, I can give you a small hint.  When the pK_a values are well-separated, one can treat each dissociation independently to a good approximation, which makes the calculation simpler in many situations.  That approximation gets worse when the pK_a values are close together.  For which polyprotic acid is this likely to be a good approximation, and for which polyprotic acid is this likely to be a poor approximation.

[Borek]

Compare http://www.titrations.info/acid-base-titration-curve-calculation (and things linked there).

If it is a mixture the only viable way is to use some software designed to find acid/base equilibria. ChemBuddy offers tools that will be helpful.