[Kasslyn]

I've been working on this lab report all evening and I can't seem to get a reasonable answer. I know the alcohol content in vodka is higher that 1.37%...

To look at exactly what I had to do, there is a breakdown of the experiment here --> link

I followed the assignment at the end and wound up with a whooping 1.37%. Obviously, something's wrong.

A breakdown of the information that I gather is as follows.

For K2Cr2O7
Reagent mass = 4.000 g
Solution volume = 100.00 mL
Concentration = 0.1360 mol/L

For Fe(NH4)2(SO4)2 * 6H2O
Reagent mass = 4.000 g
Solution volume = 100.00 mL
Concentration = 0.1020 mol/L

For the Vodka
Reagent volume = 2 mL
Solution volume = 100.00 mL
Concentration = ?

I solved the concentration with the basic moles divided by litres.


For the second part of the lab, I mixed 5.000 mL of diluted vodka, 3 mL of 6.0 M H2SO4, and 5.000 mL of K2Cr2O7. This solution was then titrated with 10.35 mL of Fe(NH4)2(SO4).

I followed the assignment to give me an outline that I thought would structure my calculations and help me find an answer

Code: [Select]

1. Record and calculate the following:

     (a) volume of dichromate solution added to the flask (mL):

      (b) moles of dichromate ion:

      (c) volume of iron(II) solution delivered from the burette (mL):

      (d) moles of iron(II) ions delivered from the burette:

      (e) moles of excess dichromate ions reacted with the iron(II) ions (remember the ratio in which they react!):

      (f) moles of dichromate that originally oxidized the ethanol in the vodka, by subtraction of (e) from (b):

      (g) moles of ethanol in the vodka (remember the ratio in which the dichromate ions react with the ethanol molecules):

2. The amount of alcohol in a drink is typically reported as the percent alcohol by volume. To find this:

      (a) calculate the mass of alcohol (ethanol) in the sample solution (g):

      (b) record the volume of vodka used in the test samples (mL):

      (c) Given the density of ethanol as 0.7893 g/mL, calculate the percent alcohol by volume as:

% by volume= [(mass of ethanol) / (density of ethanol)] / volume of vodka x 100%
1. a) 5.000 mL or 0.005 L
b) 0.00068 moles (Cr2O7)2-
c) 0.01035 L
d) 0.0010557 moles Fe2+
e) 0.00017595 mol (Cr2O7)2-
f) 0.00050405 mol (Cr2O7)2- reacted with Ethanol
e) 0.000756 mol Ethanol

2. a) 0.034830863 g Ethanol
b) 2 mL
c) 0.027492 / 2 = 1.37%



Can someone help show me what I did wrong?

[Kasslyn]

Aha! Got it. I was taking the percent of 5 mL of the 100 mL solution. Multiplied the mass by 20 to get the mass for the 100 mL solution, then divided by ethanol density, then divided by 2 mL. Got 44.12%. Sorry!