I would like some help with the following problem question which i
need to have answered over the christmas period. I have tried to use
a previous example however i cannot seem to get to the same answer
and so cannot apply the formula correctly to the question i need
answering.

This is the question i would like help with (a):

Nitrobezene (C6H5NO2, mol mass = 123.11g) can be reduced to
phenylhydroxylamine (C6H5NHOH) at a constant potential of -0.96v (vs
SCE) at a mercury electrode. In a coulometric determination using
the above conditions of 0.350g of an organic mixture which contained
nitrobenzene was dissolved in 100.0mL of methanol and electrolysis
performed. The reaction was complete after exactly 30 minutes. The
total charge passed through the cell was 26.74C. Calculate the
percentage (w/w) of nitrobenzene in the sample. (F= 96485 C.mol-1.)

The question which i am supposed to use as an example is as follows
(b):

Nitrobenze in a sample was determined by controlled potential
coulometry based on the electrochemical reduction of nitrobenze to
phenylhydroxylamine. The sample (250mg) was dissolved in 100mL of
methanol and electrolysis performed at a constant potential of -.96 V
vs SCE applied to a mercury electrode. A charge of 32.5 C was
required for a complete reaction. Calculate the percentage by weight
of nitrobenzene in the sample. F = 96485C mol-1, mol mass
nitrobenzene = 129.06g).

The answer to (b) is as follows:

The reduction of nitrobenzen proceeds according to:

C6H5NO2 + 4H+ == C6H5NHOH + H20

number of mols nitrobenzene in sample:

n = charge / 4.96485

= 32.5 / 4.96485 = 8.42 x 10-5 (ten to the
power minus 5).

Mass nitrobenze in sample:

m = 8.42 . 10-5 x 129.06 = 0.0109g

Percentage by weight of nitrobenzene in sample

% w/w = 100 x 0.0109/ 250 . 10-3 = 4.35.

My main problem is i cant get the same answer for the first line when
i calculate 32.5/ 4.96485 the answer is always 6.54601851.

Can you please offer some advise

Thankyou Sara



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