Question

While running the titration of your unknown monoprotic acid with NaOH, the pKa of the acid is 3.14. Considering the data in the following table, at what volume of NaOH would you need to reach the equivalence point? units mL, 1 decimal place

Volume NaOH added (mL)	pH
0	2.46
1.0	2.55
2.0	2.65
3.0	2.80
4.0	2.97
5.0	3.14
6.0	3.77
7.0	4.07
8.0	4.41

Answer 1

If NaOH is added to weak monoprotic acid then buffer will be formed. The pH of buffer is equal to pKa of weak acid at half equivalence point. That means if half of the acid is neutralized by NaOH then pH is equal to pKa. This is because at this point the amount of acid remains is equal to the amount of conjugate base produced.

pH= pKa + log(conjugate base/acid)

From the table given at 5.0 mL volume of NaOH the pH of buffer is equal to pKa of weak acid. So this is the half equivalence point.

Hence the volume of NaOH is needed to reach the equivalence point = 2*5.0 mL

                                                                                                   = 10.0 mL   NaOH