Question

How do you calculate pH diprotic acid?

Answer 1

I would not typically teach this to my high school students, so I looked around and found a great explanation on you tube.
Since, in a polyprotic acid the first hydrogen will dissociate faster than the others, If the Ka values differ by a factor of 10 to the third power or more, it is possible to approximately calculate the pH by using only the Ka of the first hydrogen ion. For example:

Pretend that `H_2X` is a diprotic acid. Look up on a table the Ka1
for the acid.

If you know the concentration of the of the acid, say it is 0.0027M and the `Ka_1` is `5.0 x 10^(-7)`. Then you can set up your equation as follows;
`H_2X` --> `H^(+1)` + `HX^(-1)` with `Ka_1` = `5.0x10^(-7)` Using the formula: Ka=(products) / (reactants):

`5.0x10^(-7)` = (`x^2`)/(0.0027) Then solve for x and you have your hydrogen ion concentration. Since pH = -log of hydrogen ion concentration, you can now calculate the pH.