Question

A sample of sodium carbonate unknown is dissolved in 50.0 mL of water. After performing experiment 3, the student determines that the second equivalence point occurs at 32.86 mL of 0.1115 M HCl. What is the theoretical pH of the solution after 16.43 mL of HCl has been added. Report your answer to two significant figures. (HINT - refer to the lab procedure and posted notes for the correct way to approach this problem)

Answer 1

volume HCl to reach second equivalence point = 32.86 ml

Volume to reach first equivalence point = 32.86 ml/2 = 16.43 ml

So we have to calculate pH at Ist equivalence point

pH at first equivalence point for a diprotic acid = 1/2(pKa1 + pKa2)

taking values for pKa1 and pKa2 for H2CO3

So, theoretical pH,

pH after adding 16.43 ml HCl = 1/2(6.35 + 10.33)

                                                = 8.34