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dem82 [27]
3 years ago
12

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 sam

ple 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.
Chemistry
1 answer:
MAXImum [283]3 years ago
4 0

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%.

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