Question

The leaves of the rhubarb plant contain high concentrations of diprotic oxalic acid (HOOCCOOH) and must be removed before the stems are used to make rhubarb pie. If pK_a1 = 1.23 and pK_a2 = 4.19, what is the pH of a 0.0205 M solution of oxalic acid?

Answer 1

In the case of a strong acid, the dissociation of the acid occurs completely and we do not need to take the dissociation constant into consideration for calculating the pH. That is not true in the case of oxalic acid, which is a weak diprotic acid. Its dissociations can be written as follows:

From the given data, pKa1= 1.23 and pKa2= 4.19

pKa = -log(Ka)

Using the formula,

Ka1= 5.88x10-2 ; Ka2 = 6.45x10-5

In equation 1,

where x is the concentration of the product molecules after dissociation.