Question

Carbonic acid, H2CO3, is a weak diprotic acid with pKa values of 4.0 and 10.0. Its conjugate base forms are bicarbonate and carbonate. Use this information to answer the questions.

d) What is the pH of a buffer made by mixing 100 mL of 0.1 M NaHCO3 and 10 mL of 0.1 M Na2CO3? Answer= pH=9

What is the minimum volume of 1.0 M HCl that you would need to add to 100 mL of the solution in Part (d) to get to pH = pKa? <-- Help with this part

Answer 1

d) pH = pka2 + log(Na2CO3/NaHCO3)

   pka2 of H2CO3 = 10

no of mol of NaHCO3 = 100*0.1 = 10 mmol

no of mol of Na2CO3 = 10*0.1 = 1 mmol

pH = 10+log(1/10)

    = 9

after addition of HCl, pH is decreases,

no of mol of HCl must add = y

After addition of 2 mmol HCl , Na2CO3 converts in to NaHCO3

so that, no of mol of NaHCo3 in the solution = 11 mmol

pH = pka1 + log(NaHCO3/H2CO3)

pka1 of H2CO3 = 4

4 = 4 +log((11-x)/x)

x =5.5

total no of mol of HCl must add = y = 5.5+2 = 7.5 mmol

volume of HCl must add = 7.5/1 = 7.5 ml