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May 3, 2015

Homework Help: Analytical chemistry

Posted by Gloria on Tuesday, July 30, 2013 at 11:09am.

A 5ml portion of table wine was diluted to 100 ml in a volumetric flask. The ethanol in a 20 ml aliquot was distilled into 100 ml of 0.05151M K2Cr2O7. heating completed oxidation of the alcohol to acetic acid:
Ch3Ch2OH + 2Cr2O7(2-) +16H(+) -> 3Ch3COOH + 4Cr(3+) + 11H2O following which the excess dichromate was titrated with 14.42mL of 0.02497M Fe(II). Calculate the weight-volume percentage of ethanol.
(RESULT: 11,72%)

So i tried doing this problem and again, i don't know how to continue (if i even started right)

n(K2Cr2O7)=0.1L*0.05151M=0,005151 mol
n(EtOH)=n(K2Cr2O7)/2

Cr2O7(2-) + 3Fe(2+) -> Cr(3+) + 3Fe(3+)
n(Fe2+)=0,01442L*0,02497M=0,0003601 mol
n(Fe2+)=n(Cr2O72-)*3
n(Cr2O72-)'=0,00012 mol

n(K2Cr2O7)=0,005151-0,00012=0,005031 mol
m(EtOH)=(0,005031/2)*46,07=0,11589 g

now i don't know how to continue :S

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