How do you calculate pH diprotic acid?

1 Answer

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.