I\'m having a hard time with this complicated lab, could someone please help and

Question

I'm having a hard time with this complicated lab, could someone please help and give me a thorough explanation?

Short Answer

Oxidation-Reduction Titration

Experiment 1: Prepare the Materials

Data Analysis

Calculate the concentration of the dichromate ion in the first volumetric flask.

Calculate the concentration of the iron (II) ion in the second volumetric flask.

Experiment 2: Titrate the Vodka Sample

Lab Results

Record the following lab data in the table below. If you had to repeat one of the titrations, disregard the value that was different.

14.00 - 15.13 mL

Data Analysis

Record and calculate the quantities in the table below using the data from your dichromate titrations. Use an average value for the volume of iron (II) solution used in the titration. If one of your values is very different, and you had to perform the titration three times, disregard the value that was very different when computing the average.

  The amount of alcohol in a drink is typically reported as percent alcohol by volume. Volume percent or volume/volume percent (% v/v) most often is used when preparing solutions of liquids. Volume percent is defined as:
% v/v = Vsolute/Vsolution × 100
Find the percent alcohol (ethanol) by volume for the vodka used in the lab by following the steps outlined in the table below.

Conclusions

The Grey Moose vodka tested in this lab reports a percent alcohol by volume of 40.0% on its label. How does your value compare to the reported one? If the values are different, give one possible experimental error that might have contributed to the difference.  

Potassium permanganate is another strong oxidizing substance similar to potassium dichromate. An acidic solution of purple permanganate ions can get reduced to colorless Mn2 ions in the presence of ethanol. Write down the redox reaction between permanganate and ethanol, and balance it using the half-reaction method.

Besides vodka, there are other colorless alcohol-containing beverages that can be titrated following the procedure in your lab. Given the average values for the percent alcohol by volume listed in the table below, which beverage do you expect to use the least amount of iron (II) standard solution during the titration? Assume all lab procedures stay the same.

NOTES:

Experiment 1: Prepare the Materials

(all are dissolved in water filled to 100 mL mark)

Flask 1: (4 g) Standard Potassium Dichromate Solution (K2Cr2O7)

Flask 2 & 3: (4 g) Standard Iron(II) Solution (Fe(NH4)2(SO4)2 × 6H2O)

Flask 4: (2 ml) 2% Vodka Solution

Experiment 2: Titrate the Vodka Sample

Part 1: Oxidize the Ethanol in Vodka

Add 5.00 mL of 2% vodka solution from the volumetric flask to the Erlenmeyer flask.

Take water from the Materials shelf and add 35.00 mL to the Erlenmeyer flask. Note that this further dilutes the vodka sample by a factor of eight. The ethanol concentration is now 1/8th of 2%, or 0.25% of the original ethanol concentration of the bottled vodka.

Acidify the vodka solution in the Erlenmeyer flask. Take the sulfuric acid (H2SO4) solution from the Materials shelf and add 5.00 mL to the Erlenmeyer flask.

Add 5.00 mL of the standard potassium dichromate solution from the volumetric flask (Flask 1) to the Erlenmeyer flask. This is enough to reduce all of the ethanol in the vodka and leave an excess of dichromate ions. Note that the solution has turned bright green. This is the color of the reduced Cr3+ ions.

Part 2: Coarse Titration

Fill the burette with 50 mL of the standard iron(II) solution

Take the redox indicator, sodium diphenylamine sulfonate, from the Materials shelf and add 0.50 g to the Erlenmeyer flask. In the presence of the excess dichromate ions, the solution turns a deep purple. (it's purple now)

Perform a coarse titration

As the iron(II) is added, the dichromate ions (Cr2O72–) are reduced to Cr3+ ions. At the end point of the titration, there are no dichromate ions left. The redox indicator becomes colorless, and the dark purple color suddenly disappears, leaving the solution bright green again. Recall that bright green is the color of the Cr3+ ions.

14.00 mL dispensed before

15.13 GREEN AGAIN

Part 3: Fine Titration

Set up the titration as before:

Add 5.00 mL of diluted vodka, 35.00 mL water, 5.00 mL of sulfuric acid, 5.00 mL of the standard potassium dichromate solution, and 0.50 g sodium diphenylamine sulfonate to an Erlenmeyer flask.

And refill burrette

14.00 mL again

14.15 GREEN

Explanation / Answer

In given experiment first we find

Conc.of K2Cr2O7 in flask 1 = moles of K20.Cr2O7/vol. Of solution in Litres = 4/294/100/1000 = 0.136M

Conc. Of (NH4)2Fe(SO4)2.6H2O , Mohr salt = 4/392/0.1

= 0.102M

No. Of moles of dichromate present in 5ml volume

= 0.136*5 = 0.68millimoles

After addition of ethanol into it , it was titrated against mohr salt by using redox indicator.

Vol of mohr salt used to reach the end point=14.15ml

Cr2O72- + 6 Fe2+ + 14H+ ------->2 Cr3+ + 6 Fe3+ +7H2O

6 * Moles of dichromate = Moles of Fe(II). ( ratio of moles from equation)

6* moles of dichromate left after oxidise ethanol

= M* V of Fe(II) = 0.102*14.15/6 = 0.24millimoles

Moles of dichromate used to oxidise ethanol = 0.68-0.24

= 0.44millimoles

Now ethanol and dichromate reacts in the ratio 3:2

Moles of ethanol/moles of dichromate= 3/2

So moles of ethanol present = 3*0.44/2 =0.66millimoles = 0.66*10-3moles

In dil. vodka , moles of ethanol=0.66*10-3 moles

Mass of ethanol = moles* molar mass

= 0.66*10-3 *46.07= 30.4 * 10-3 g

Volume of ethanol = mass/ density

= 30.4*10-3 /0.7893 = 38.5*10-3 ml

In 5ml dilute vodka , ethanol present is 38.5*10-3 ml

v/V% = 38.5 * 10-3 *100 /5= 0.77%(v/V)

Dilution factor in 2ml to 100ml is 100/2 =50

v/V% in original vodka solution= 50* 0.77=38.5%

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