Question

6. Identify the titrant in each pair that would provide a greater pH change about the equivalence point in the titration of a 0.100 M NaOH solution. Explain your reasoning for your choice for each pair. a) 0.10 M benzoic acid (C6H5CO2H) or 0.10 M chloroacetic acid (CICH2CH2CO2H) (2 pts) b) 0.10 M hydrofluoric acid (HF) or 0.10 M formic acid (HCO2H) (2 pts)

Answer 1

In acid base titration, generally there is a titrant ( an unknown which concentration to be calculated ) and a titrand (a solution with known concentration) is taken.

Generally our titrants are acids, which pH are to be calculated, taken in a flask or beaker. And the titrand is a strong base which is taken in a burrette and added to the acid in flask dropwise untill it reaches the equivalance point.

Now in the question, our titrand is a strong base NaOH.

Again equivalance point is that point where all the NaOH added have been completely used in neutralizing the acid present. Thus at this point, all the acids are converted to its salt of conjugate base. Thus will form a buffer

a-

Here both the given acids C6H5-CO2H and ClCH2CH2CO2H are weak acids. Now pKa for C6H5-CO2H = 4.20 and pKa for ClCH2CH2CO2H = 2.87

We know lesser the pKa, stronger is the acid. Thus between these pair, ClCH2CH2CO2H is a stronger acid than C6H5-CO2H.

The pH of a buffer is calculated by Henderson Hasselbalch equation-

pH = pKa + log [salt]/[acid]

If we consider for a particular acid, then its pKa value is always same. That means the change in pH depends upon the change in concentration of salt and acid. Higher the [salt] and lower the [acid] will produce a higher log [salt]/[acid] value. Thus a higher change in pH value is observed.

Now generally weaker acids corresponds to greater changes when strong bases are added. Thus they will provide a greater change in pH.

On this basis, C6H5-CO2H will produce greater change in pH on addition of NaOH

2- Similarly Here both the given acids HF and HCO2H are weak acids. Now pKa for HF = 3.1 and pKa for HCO2H = 3.74

That means  HCO2H is the weak acid in the pair. Hence HCO2H will produce greater change in pH on addition of NaOH