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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.