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# How do you calculate pH diprotic acid?

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 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.