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Oxidation-Reduction Titration

***Sulfuric acid used was 6 M

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

PROCEDURES:

Experiment 1: Prepare the Materials

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

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

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

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

Complete the solution by filling the volumetric flask to the100.00 mL mark with water from the Materials shelf by checking the\"Fill To Mark\" box.

Double-click on the volumetric flask to open a propertieswindow. Then, rename the volumetric flask as \"Standard PotassiumDichromate 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 Materialsshelf and add 4.00 g to each empty volumetric flask.

Take water from the Materials shelf and add 30.00 mL to eachvolumetric flask to dissolve the compound and release the water ofhydration.

Complete both solutions by filling the volumetric flask to the100.00 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 2.00mL to the last empty flask. Fill with water from the Materialsshelf by checking the \"Fill To Mark\" box. Thevodka has now been diluted to 1/50th, or 2%, of its originalethanol concentration.

Rename the volumetric flask containing the vodka as \"2% VodkaSolution\".

Experiment 2: Titrate the Vodka Sample

Part 1: Oxidize the Ethanol in Vodka

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

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

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

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

Add 5.00 mL of the standard potassium dichromate solution fromthe volumetric flask to the Erlenmeyer flask. This is enough toreduce all of the ethanol in the vodka and leave an excess ofdichromate ions. Note that the solution has turned bright green.This is the color of the reduced Cr³⁺ ions. Record theseobservations in your Lab Notes. Remember to pressSave Notes each time you add more notes.

Part 2: Coarse Titration

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

Take the redox indicator, sodium diphenylamine sulfonate, fromthe Materials shelf and add 0.50 g to the Erlenmeyer flask. In thepresence of the excess dichromate ions, the solution turns a deeppurple.

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

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

As the iron(II) is added, the dichromate ions(Cr₂O₇^2–) are reduced toCr³⁺ ions. At the end point of the titration, there areno dichromate ions left. The redox indicator becomes colorless, andthe dark purple color suddenly disappears, leaving the solutionbright green again. Recall that bright green is the color of theCr³⁺ ions.

Record both the last burette volume that the solution was darkpurple and the burette volume at which the solution first appearedgreen again in your Lab Notes.This gives the range in which the titration will end. Remember topress Save Notes.

Discard just the Erlenmeyer flask in the recycling binunderneath 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 ofsulfuric acid, 5.00 mL of the standard potassium dichromatesolution, and 0.50 g sodium diphenylamine sulfonate to anErlenmeyer flask.

Connect the Erlenmeyer flask to the lower half of theburette.

Note the current volume of standard iron(II) solution in theburette. Add to it from the volumetric flask on the workbench sothat the volume is 50.00 mL again. Record the initial burettereading for the amount dispensed in your LabNotes.

Click and hold the black knob of the burette to quickly addenough standard iron(II) solution to just get into the range of thecoarse titration (the first number you recorded), but still havethe solution in the flask appear dark purple. This is near, but notyet at, the titration's end point.

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

Place the Erlenmeyer flask in the recycling bin beneath theworkbench.

Repeat the fine titration once more, and record the results inyour Lab Notes. If the results from the two finetitrations do not closely agree, perform a third fine titration todetermine 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 firstvolumetric flask.

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

Experiment 2: Titrate the Vodka Sample

Lab Results

Record the following lab data in the table below. If you had torepeat one of the titrations, disregard the value that wasdifferent.

(a) volume of potassiumdichromate solution added to the Erlenmeyer flask in mL	5.000 mL
(b) coarse titrationvolume 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 finetitration	14.15 mL
(d) volume of iron (II)solution delivered from the burette in mL during the second finetitration	14.14 mL
(e) average volume of iron(II) solution used in the fine titrations	14.145 mL
(f) the color of theanalyte solution at the end point of the titration	Bright green
(g) the color of theanalyte solution after adding the indicator	Deep purple

Data Analysis

Record and calculate the quantities in the table below using thedata from your dichromate titrations. Use an average value for thevolume of iron (II) solution used in the titration. If one of yourvalues is very different, and you had to perform the titrationthree times, disregard the value that was very different whencomputing the average.

(a) volume ofpotassium dichromate solution added to the Erlenmeyer flask inmL	5.000 mL
(b) moles of dichromateion 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 excessdichromate ions that reacted with the iron (II) ions (remember thatthe ratio in which they react is 1 dichromate : 6 iron (II))
(f) moles of dichromatethat reacted with the ethanol in the vodka (Subtract excessdichromate ions that reacted with the iron (II) ions from theoriginal moles of dichromate ion present.)
(g) moles of ethanol inthe 5 mL diluted vodka sample according to the stoichiometric ratioof 2 dichromate ions to 3 ethanol molecules

  The amount of alcohol in a drink is typicallyreported as percent alcohol by volume. Volume percent orvolume/volume percent (% v/v) most often is used when preparingsolutions 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 inthe lab by following the steps outlined in the table below.

(a) given the molar mass of ethanol of46.07 g/mol, calculate the mass of alcohol (ethanol) in the testedsample solution
(b) given the density of ethanol of0.7893g/ml, find the volume in mL of ethanol present in the dilutedvodka solution
(c) record the volume of vodka used inthe experiment in mL
(d) find the percent alcohol by volume(% v/v) in the diluted vodka solution
(e) the diluted vodka solution wasprepared by diluting 2.00 mL vodka to 100.00 mL. Calculate thedilution factor used (N:1)
(f) multiply the percent alcohol byvolume in the diluted vodka solution by the dilution factor toobtain the % v/v alcohol in the original vodka solution

Conclusions

The Grey Moose vodka tested in this lab reports a percentalcohol by volume of 40.0% on its label. How does your valuecompare to the reported one? If the values are different, give onepossible experimental error that might have contributed to thedifference.  

Potassium permanganate is another strong oxidizing substancesimilar to potassium dichromate. An acidic solution of purplepermanganate ions can get reduced to colorless Mn²ionsin the presence of ethanol. Write down the redox reaction betweenpermanganate and ethanol, and balance it using the half-reactionmethod.

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

	% alcohol byvolume
White rum	37.0%
Vermouth	18.0%
White whine	12.0%

LAB NOTES:

Solution turned bright green upon adding the standard potassiumdichromate.

Initial burette reading: 50 mL

(Solution turned deep purple after adding sodium diphenylaminesulfonate)

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