# Chemistry [back-titration] - please check my sol'n

A 7.00 gram sample containing the mineral tellurite was dissolved then treated with 70.0 mL of 0.03114 M K2CrO7.

3 TeO2 + Cr2O7 ^2- + 8 H+ -> 3 H2TeO4 + 2 Cr3+ + H2O

When reaction is complete, the excess Cr2O7 2- required a 14.05 mL back titration with 0.1135 M Fe2+. Calculate the % TeO2 in the sample. (The correct answer is 13.09 %)

I tried doing calculations; this is my solution: I believe ferrous ion and dichromate ion are in 1:1 ratio in their reaction, so mol of Fe is equal to mol Cr2O7, which is
0.01405 L x 0.1135 M = 0.0015947 mol (excess mol Cr2O7)
So the actual mol Cr2O7 reacted is
(0.070 mL x 0.03114 M) - 0.0015947 mol = 5.851 x 10^-4 mol
moles of TeO2 that reacted is
5.851 x 10^-4 mol Cr2O7 * (3 TeO2 / 1 Cr2O7) = 1.7553 x 10^-3 mol TeO2
molar weight of TeO2 = 127.6 + 2*16 = 159.6 g/mol
actual weight of TeO2 = 1.7553 x 10^-3 * 159.6 = 0.2801 g
% TeO2 in sample = 0.2801 / 7 * 100 = 4.00 %

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1. One mistake is the 1:1 ratio for Fe^2+ vs Cr2O7^2-. The partially balanced equation is
6Fe^2+ + Cr2O7^2- ==> 2Cr^3+ + 6Fe^3+
A second error is I have the molar mass TeO2 as 159.6
I worked the problem and 13.09% TeO2 is correct. I didn't go through the rest of your work but it looks ok at first scrutiny. Work it again with the two corrections I've noted and see if you don't get the right answer. If not repost and show what you've done and I'll go through your work in detail. Again, however, I worked the problem from scratch and came up with 13.09%,
By the way, that problem you posted the other night on fusion of 1H1 to 1H2, I posted a response. I think the error was you had transposed the numbers in the mass of 1H1. I might be able to find it if you haven't seen my response.

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2. I get it now. I didn't think about the reaction of Fe^2 and Cr2O7^2-. I thought they would produce FeCr2O7, so I thought they're in 1:1 ratio. Yes, I got 159.6 g/mol as weight of TeO2 (as what I've typed above). :)

Yes, I've seen the post about the nuclear chem problem, but I double checked the question and I didn't have a typo; the values from the book are really what I typed in the problem.

Anyway, thank you very much for your help. :D

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