Question

Oxidation-Reduction Titration

***Sulfuric acid used was 6 M

***All Lab notes are at the bottom, procedure included for clarity... PLEASE HELP WITH THE SHORT ANSWER:)

PROCEDURES:

Experiment 1: Prepare the Materials

Take four 100.00 mL volumetric flasks from the Containers shelf and place them on the workbench.

In one flask, prepare a standard solution of potassium dichromate (K₂Cr₂O₇):

Take potassium dichromate from the Materials shelf and add 4.00 g to the volumetric flask.

Take water from the Materials shelf and add 30.00 mL into the volumetric flask to dissolve the dichromate compound.

Complete the solution by filling the volumetric flask to the 100.00 mL mark with water from the Materials shelf by checking the "Fill To Mark" box.

Double-click on the volumetric flask to open a properties window. Then, rename the volumetric flask as "Standard Potassium Dichromate Solution".

In two of the empty flasks, prepare a standard solution of iron (II) ammonium sulfate hexahydrate (Fe(NH₄)₂(SO₄)₂ × 6H₂O):

Take iron(II) ammonium sulfate hexahydrate from the Materials shelf and add 4.00 g to each empty volumetric flask.

Take water from the Materials shelf and add 30.00 mL to each volumetric flask to dissolve the compound and release the water of hydration.

Complete both solutions by filling the volumetric flask to the 100.00 mL mark with water from the Materials shelf by checking the "Fill To Mark" box.

Rename the volumetric flasks as "Standard Iron(II) Solution". Both flasks will have the same name.

Take the Grey Moose vodka from the Materials shelf and add 2.00 mL to the last empty flask. Fill with water from the Materials shelf by checking the "Fill To Mark" box. The vodka has now been diluted to 1/50th, or 2%, of its original ethanol concentration.

Rename the volumetric flask containing the vodka as "2% Vodka Solution".

Experiment 2: Titrate the Vodka Sample

Part 1: Oxidize the Ethanol in Vodka

Take a 150.00 mL Erlenmeyer flask from the Containers shelf and place it on the workbench.

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 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. Record these observations in your Lab Notes. Remember to press Save Notes each time you add more notes.

Part 2: Coarse Titration

Take a burette from the Containers shelf and place it on the workbench. Fill the burette with 50 mL of the standard iron(II) solution. Record the initial burette reading for the amount of volume dispensed in your Lab Notes. Before dispensing any liquid, the amount dispensed should read 0 mL.

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.

Place the Erlenmeyer flask on the lower half of the burette to connect to flask and burette.

Perform a coarse titration by adding large increments of the standard iron(II) solution from the burette. To do this, press and hold the black knob at the bottom of the burette until the solution turns suddenly from intense, dark purple to green. Each time you add the standard iron(II) solution, check the volume dispensed from the burette by hovering over the burette and reading the gray tool tip. You will need to know this value.

As the iron(II) is added, the dichromate ions (Cr₂O₇^2–) 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.

Record both the last burette volume that the solution was dark purple and the burette volume at which the solution first appeared green again in your Lab Notes. This gives the range in which the titration will end. Remember to press Save Notes.

Discard just the Erlenmeyer flask in the recycling bin underneath the workbench.

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.

Connect the Erlenmeyer flask to the lower half of the burette.

Note the current volume of standard iron(II) solution in the burette. Add to it from the volumetric flask on the workbench so that the volume is 50.00 mL again. Record the initial burette reading for the amount dispensed in your Lab Notes.

Click and hold the black knob of the burette to quickly add enough standard iron(II) solution to just get into the range of the coarse titration (the first number you recorded), but still have the solution in the flask appear dark purple. This is near, but not yet at, the titration's end point.

Add standard iron(II) solution in small increments, down to one drop at a time, until the addition of just one more drop causes the solution in the flask to turn green. Record the final burette reading for the amount of volume dispensed in your Lab Notes.

Place the Erlenmeyer flask in the recycling bin beneath the workbench.

Repeat the fine titration once more, and record the results in your Lab Notes. If the results from the two fine titrations do not closely agree, perform a third fine titration to determine which of the first two was done incorrectly.   

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.

(a) volume of potassium dichromate solution added to the Erlenmeyer flask in mL	5.000 mL
(b) coarse titration volume of iron (II) solution range in mL	34.76-37.74 mL
(c) volume of iron (II) solution delivered from the burette in mL during the first fine titration	14.15 mL
(d) volume of iron (II) solution delivered from the burette in mL during the second fine titration	14.14 mL
(e) average volume of iron (II) solution used in the fine titrations	14.145 mL
(f) the color of the analyte solution at the end point of the titration	Bright green
(g) the color of the analyte solution after adding the indicator	Deep purple

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.

(a) volume of potassium dichromate solution added to the Erlenmeyer flask in mL	5.000 mL
(b) moles of dichromate ion added to the Erlenmeyer flask
(c) average volume of iron (II) solution delivered from the burette in mL
(d) moles of iron (II) ions delivered from the burette
(e) moles of excess dichromate ions that reacted with the iron (II) ions (remember that the ratio in which they react is 1 dichromate : 6 iron (II))
(f) moles of dichromate that reacted with the ethanol in the vodka (Subtract excess dichromate ions that reacted with the iron (II) ions from the original moles of dichromate ion present.)
(g) moles of ethanol in the 5 mL diluted vodka sample according to the stoichiometric ratio of 2 dichromate ions to 3 ethanol molecules

  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 = V_solute/V_solution  × 100
Find the percent alcohol (ethanol) by volume for the vodka used in the lab by following the steps outlined in the table below.

(a) given the molar mass of ethanol of 46.07 g/mol, calculate the mass of alcohol (ethanol) in the tested sample solution
(b) given the density of ethanol of 0.7893g/ml, find the volume in mL of ethanol present in the diluted vodka solution
(c) record the volume of vodka used in the experiment in mL
(d) find the percent alcohol by volume (% v/v) in the diluted vodka solution
(e) the diluted vodka solution was prepared by diluting 2.00 mL vodka to 100.00 mL. Calculate the dilution factor used (N:1)
(f) multiply the percent alcohol by volume in the diluted vodka solution by the dilution factor to obtain the % v/v alcohol in the original vodka solution

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 Mn²ions 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.

	% alcohol by volume
White rum	37.0%
Vermouth	18.0%
White whine	12.0%

LAB NOTES:

Solution turned bright green upon adding the standard potassium dichromate.

Initial burette reading: 50 mL

(Solution turned deep purple after adding sodium diphenylamine sulfonate)

Coarse Titration:

First dispense

Volume: 46.93 mL

Volume dispensed: 3.07 mL

Second dispense

Volume: 44.07 mL

Volume dispensed: 5.93 mL

Third dispense

Volume: 40.91 mL

Volume dispensed: 9.09 mL

Fourth dispense

Volume: 37.74 mL

Volume dispensed: 12.26 mL

Fifth dispense- END POINT REACHED

Volume: 34.76 mL

Volume dispensed: 15.24 mL

Fine Titration 1

End point volume: 35.85 mL

Volume dispensed: 14.15 mL

Fine Titration 2:

End point volume: 35.86 mL

Volume dispensed: 14.14 mL

Answer 1

Calculate the molarity concentration of potassium dichromate, K2Cr2O7, in the firstvolumetric flask.This is also the concentration of dichromate ion

Molarity of.k2cr2o7

= (4 k2cr2o7/1) x ( 1mol k2cr2o7 /298.818 g k2cr2o7 ) x( 1 /0.100 l k2cr2o7)

=0.136 M

Calculate the molarity concentration of Fe(NH 4 ) 2 (SO 4 ) 2 · 6H 2 O in the second two volumetric flasks. This is also the concentration of Fe 2+ ion

molarity concentration of Fe(NH 4 ) 2 (SO 4 ) 2 · 6H 2 O =

= 4.00g/ 1 x 1 mol/ 392.13 x 1/0.100

=0102 M

ecord 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.

(a) volume of potassium dichromate solution added to the Erlenmeyer flask in mL	5.000 mL
(b) moles of dichromate ion added to the Erlenmeyer flask	0.680 mmol
(c) average volume of iron (II) solution delivered from the burette in mL	14.10 mL
(d) moles of iron (II) ions delivered from the burette	1.44 mmol
(e) moles of excess dichromate ions that reacted with the iron (II) ions (remember that the ratio in which they react is 1 dichromate : 6 iron (II))	0.240 mmol
(f) moles of dichromate that reacted with the ethanol in the vodka (Subtract excess dichromate ions that reacted with the iron (II) ions from the original moles of dichromate ion present.)	0.440 mmol
(g) moles of ethanol in the 5 mL diluted vodka sample according to the stoichiometric ratio of 2 dichromate ions to 3 ethanol molecules	0.660 mmol