Question

For polyprotic acids and bases when determining the pH what does it mean and generally how do you calculate the pH of a diacid, intermediate and dibase?

Answer 1

FOR DIACID

Pretend that H2X is a diprotic acid. Look up on a table the Ka1
for the acid.

If we know the concentration of the of the acid, say it is 0.0027M and the Ka1 is 5.0x10−7. Then we can set up your equation as follows;
H2X --> H+1 + HX−1 with Ka1 = 5.0x10−7 Using the formula: Ka=(products) / (reactants):

5.0x10−7 = (x2)/(0.0027) Then solve for x and we have our hydrogen ion concentration. Since pH = -log of hydrogen ion concentration, we can now calculate the pH.

DIBASE

In the above scenario if we calculate the [OH-] ion and the pOH = -log OH ion concentration and pH+pOH=14.

By this we can calculate pH of dibase

INTERMEDIATE

If we know the concentration of H ion the we can simply calculate the pH. Since pH = -log of hydrogen ion concentration, we can now calculate the pH.