Question

Citric acid, C₃H₅O(COOH)₃, which is present in citrus fruits, is a triprotic acid. Calculate the concentration of the citrate, C3H5O(COO)₃^3- , of a 0.040M solution of citric acid. Ka,1 = 7.4 x10^-4, Ka,2 = 1.7 x 10^-5, Ka,3 = 4.0x 10^-7

a. 5.08 x10^-3 M

b. 1.7x10^-5 M

c. 2.29

d. 1.33 x 10^-9 M

e. 8.87

Answer 1

Is our assumption valid? 1.7x10-5 dissociated/ 5.08x10-3 initial = 0.33% dissociation = OK!! What if we did not assume? Would the H+ from the second dissociation be enough to change the pH? We can approximate by using our x value to compare 5.08x10-3 with (5.08x10-3 + x) 5.08x10-3 + 1.7x10-5 = 0.005737, which produces a pH of 2.24, so the second dissociation does not significantly change p.

This equilibrium determines how much citrate ion is in solution, but its initial concentrations are determined by the previous two dissociations