The concentration of ethanol (C2H5OH) in aqueous solutions can be determined by reacting it with pot — Stoichiometry / Redox Titration Chemistry Question
Problem Context
The concentration of ethanol (C2H5OH) in aqueous solutions can be determined by reacting it with potassium dichromate (K2Cr2O7) in acid to produce ethanoic acid (CH3COOH) and Cr 3+ ions.
Write a balanced equation for this reaction.
Model Answer
2 × (Cr2O7 2– + 14 H+ + 6 e– 2 Cr3+ + 7 H2O)
3 × (C2H5OH + H2O → CH3COOH + 4 e¯ + 4 H+)
2 Cr2O7 2–+ 3 C2H5OH + 16 H+ 4 Cr3+ + 3 CH3COOH + 11 H2O
Describe the color change observed during this reaction.
Model Answer
orange green
A standard solution of K2Cr2O7 is prepared in the following manner:
125 mL of H2O is placed in a 250 mL volumetric flask
70 mL of concentrated H2SO4 is added while swirling and cooling under running water
0.750 g of dried K2Cr2O7 is added and the solution is diluted to the mark on the neck of the flask with H2O.
i. Account briefly for the order in which the liquid solution components are mixed (H2O, H2SO4, H2O) and for the fact the initial mixture is cooled.
Model Answer
Acid should be added to water to prevent spattering. Mixture is cooled to dissipate heat of dilution.
ii. Calculate the [Cr2O7 2–] in the 250 mL solution.
Model Answer
0.750g × (1 mol / 294.2g) × (1 / 0.250L) = 0.0102 M
0.600 mL of a popular mouthwash is diluted to 100. mL with H2O. When a 10.0 mL aliquot of this solution is titrated with the solution of K2Cr2O7 prepared in c., 20.25 mL of K2Cr2O7 are required.
i. Calculate the number of moles of ethanol in 0.600 mL of mouthwash.
Model Answer
0.02025 L × 0.0102 mol/L = 2.066 × 10–4 mol Cr2O7 2–
2.066 × 10–4 mol Cr2O7 2– × [3 mol C2H5OH]/[2(Cr2O7) 2–] = 3.099 × 10–4 mol C2H5OH in 10 mL
3.099 × 10–4 mol C2H5OH × (100 / 10) = 3.099 × 10–3 mol C2H5OH in 100 mL
ii. Determine the mass percentage of ethanol in this mouthwash (Assume the density of mouthwash to be 0.966 g•cm –3).
Model Answer
0.600 mL × (0.966 g / cm3) = 0.5796 g
3.099 × 10–3 mol × (46.7 g / 1mol) = 0.1428 g C2H5OH
24.6% Ethanol