TBR: Titration Question

[justadream]

TBR Genchem II page 290 #28

"If a 20 mL sample from flash 2 were to require a 17.5 mL aliquot of NasCr2O7 solution of unknown concentration to reach the endpoint, then what would a 20 mL sample from flask 3 require to reach equivalence?"

Flask 2: 1.52g of FeSO4 is added to enough water to make 200 mL in solution
Flask 3: 3.04g of FeSO4 is added to enough water to make 100 mL in solution

TBR's Answer: 70 mL aliquot of Na2Cr2O7

TBR Explanation (paraphrased): Flash 3 is 4 times as concentrated so you need 4x more.


I thought the answer should be 17.5*2 = 35 mL.

I don't understand why we are looking at CONCENTRATION instead of just moles. Flask 3 has twice the number of moles of FeSO4 as does Flask 2. Why does it matter how much you dilute it?

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[BerkReviewTeach]

You are not titrating all of Flask 2 and all of Flask 3. If you were, then your logic is correct. But you are taking 20 mL samples from each.

Flask 2: 20 mL x 0.010 moles/200 mL = 0.001 moles

Flask 3: 20 mL x 0.020 moles/100 mL = 0.004 moles

You have four times as many moles in 20 mL of Flask 3 solution than you have in 20 mL of Flask 2 solution.