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Finding the Concentration of Whiskey in m/v%

Add approximately 100cm^3 of water to 10cm^3 of whiskey - then fill up the flask to the 500cm^3 mark with water. Pipette 20cm^3 of this solution into a conical flask containing 70cm^3 of 0.05M acidic solution K2Cr2O7. Titrate the excess unreacted K2Cr2O7 with 24.82cm3 of 0.1268M Fe(II) solution.

Find the concentration of the cask strength whiskey in terms of %w/v.

Solution Preview

This is an experiment that uses a technique called a back titration. The concept here is that we react all the alcohol (which we assume is ethanol) with excess dichromate (K2Cr2O7) and then find out how much excess dichromate was used and then we can find out how much alcohol was there to begin with.

The titrations, however, are not straightforward, in the chemical sense, in that the relation of reactants is NOT 1:1.

The reaction equations for the reactions are found as follows

(1) 14H+ + 6Fe(II) + Cr2O7-2 --> 2Cr(3+) + 6Fe(III) + 7H2O

(2) 8 H+ + Cr2O72- + 3 C2H6O --> 2 Cr3+ + 7 H2O + 3 ...

Solution Summary

This complete solution includes calculations, answers and explanation. 432 words.