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## Main Question or Discussion Point

So I have a problem with the equation: pH = pKa + log([A-]/[HA])

The textbook states that when [H+] = Ka, there will be 50% A- and 50% HA.

However, this won't work when the coefficients of the reactants to the products are not the same. The equation only works for reactions where it is a 1:1 mole ratio of reactants to products.

So, my question is, why is it generalized and said that pH = pKa at half-equivalence?

What if we have H3PO4?

Wouldn't the overall equation be: H3PO4 + H2O <-> 3 [H3O+] +[PO4 3-]

and then that equation won't work?

The textbook states that when [H+] = Ka, there will be 50% A- and 50% HA.

However, this won't work when the coefficients of the reactants to the products are not the same. The equation only works for reactions where it is a 1:1 mole ratio of reactants to products.

So, my question is, why is it generalized and said that pH = pKa at half-equivalence?

What if we have H3PO4?

Wouldn't the overall equation be: H3PO4 + H2O <-> 3 [H3O+] +[PO4 3-]

and then that equation won't work?