Question

An unknown amount of acid can often be determined by adding an excess of base and then back-titrating the excess. A 0.3471−g sample of a mixture of oxalic acid, which has two ionizable protons, and benzoic acid, which has one, is treated with 97.0 mL of 0.1090 M NaOH. The excess NaOH is titrated with 21.00 mL of 0.2060 M HCl. Find the mass % of benzoic acid.

Answer 1

Explanation:

The given data is as follows.

       Mass of mixture = 0.3471 g

As the mixture contains oxalic acid and benzoic acid. So, oxalic acid will have two protons and benzoic acid has one proton.

This means oxalic acid will react with 2 moles of NaOH and benzoic acid will react with 1 mole of NaOH.

Hence,   moles of NaOH in 97 ml = $\frac{0.1090 \times 97}{1000}$

                                                       = $10.573 \times 10^{-3} mol$

Moles of HCl in 21.00 ml = $\frac{0.2060 \times 21}{1000}$

                                         = $4.326 \times 10^{-3}$ mol

Therefore, total moles of NaOH that reacted are as follows.

           $10.573 \times 10^{-3} mol$ - $4.326 \times 10^{-3} mol$      

                = $6.247 \times 10^{-3}$ mol

So, total 3 mole of NaOH will react with 1 mole of mixture. Therefore, number of moles of NaOH reacted with benzoic acid is as follows.

                  $\frac{6.247 \times 10^{-3}}{3}$

                    = $2.082 \times 10^{-3}$ mol

Since, molar mass of NaOH is 40 g/mol. Therefore, calculate the mass of NaOH as follows.

                    $2.082 \times 10^{-3} mol \times 40 g/mol$

                         = $83.293 \times 10^{-3}$ g

                         = 0.0832 g

Whereas molar mass of benzoic acid is 122 g/mol.

Therefore,       40 g NaOH = 122 g benzoic acid

So,            0.0832 g NaOH = $\frac{122 g}{40 g} \times 0.0832 g$

                                             = 0.253 g

Hence, calculate the % mass of benzoic acid as follows.

                      $\frac{0.253 g}{0.3471 g} \times 100$

                           = 73.10%

Thus, we can conclude that mass % of benzoic acid is 73.10%.