The nitrobenzene C 6 H 5 NO 2 , in an organic mixture was measured by reduction

Question

The nitrobenzene C6H5NO2, in an organic mixture was measured by reduction to phenylhydroxylamine at a constant potential of -0.96 V (vs SCE) applied to a mercury cathode according the reaction

C6H5NO2 + 4H+ = C6H5NHOH + H­2O

A weight of 210 mg of the sample was dissolved in 100 mL of methanol. After electrolysis of 30 minutes, the reaction was judged complete. An electronic coulometer in series with the cell indicated that the reduction required 26.74 C, Calculate the weight percentage (w/w%) of C6H5NO2 in the sample.

Explanation / Answer

Equivalent mass of C6H5NO2 = Molar mass/4 = 123/4 = 30.75

Now, charge required = 26.74 C

Thus equivalents of C6H5NO2 deposited = charge/Faraday Constant = 26.74/96500 = 2.771*10-4

Thus, mass of C6H5NO2 deposited = number of equivalents*equivalent mass = 8.521*10-3 g = 8.521 mg

Thus, mass% of C6H5NO2 = {(210-8.521)/210}*100 = 99.99%

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