Titration Q

[andafoo]

The concepts I'm asking are specifically for a problem on the free Princeton Review practice exam ... so if anyone's taken it, it is the 3rd question in the PS section.

For those who don't know the question, I can't paste the whole question with the passage... so I'll try my best:

For something to be buffered does that mean the solution (in terms of an acid/base titration) has equal concentration of acid and its conjugate base? And does this mean that they have equal moles?

The question I'm have trouble with is a titration of a galvanic cell to cell potential against ion concentration.

The question asks what volume of .1M Ce4+ solution has to be added so that a buffer of Fe2+/Fe3+ could exist.

A: 15 mL
B: 25 mL
C: 35 mL
D: 45 mL

Now the passage gives three relevant factoids (at least one's that I've extracted):

(1) the cell starts with 50 mL of 0.05 M Fe2+

(2) a rxn btween Fe2+ and Ce4+ occurs: Fe2+ + Ce4+ -> Fe3+ + Ce3+

(3) a titration curve is given with eqv point = 30 mL of .10 M Ce4+ and half eqv point at 15 mL.



The correct answer expects you to use the titration curve and determine that 15 mL needs to be added b/c that is the half-eqv point.

But my confusion arises because adding 15 mL of 0.1 M Ce4+ equates to .0015 moles of Ce4+, while there are .0025 moles of Fe2+ to begin with. That would mean .0015 moles of Ce4+ react with Fe2+ leaving .0010 moles of Fe2+ in solution with .0015 moles of Fe3+... what kind of buffer is that???

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[lixxx669]

I'm not sure what's your "eqv point" and "half eqv point"....
Basically, the buffer range is where there is a flat slop on the titration curve so you can just read the graph and choose the answer.
Besides, since a buffer system has a range, so it is not necessary that Fe3 exactly equalls Fe2, as long as they have roughly 1 to 1 ratio.

 

[andafoo]

I'm not sure what's your "eqv point" and "half eqv point"....
Basically, the buffer range is where there is a flat slop on the titration curve so you can just read the graph and choose the answer.
Besides, since a buffer system has a range, so it is not necessary that Fe3 exactly equalls Fe2, as long as they have roughly 1 to 1 ratio.

Sorry, eqv. stands for equivalence point and half equivalence point.

So when you say roughly 1 to 1 ratio you mean on a molar basis? If that is the case then the graph in the question does not match up with the numbers... now is that a mistake or an error in concept?

 

[lixxx669]

Does your "eqv point" means E=0? or all the Fe2+ turn to Fe3+?

I think it would be more helpful if you paste the graph here...

 

[andafoo]

Does your "eqv point" means E=0? or all the Fe2+ turn to Fe3+?

I think it would be more helpful if you paste the graph here...

Ehh eqv. point is a universal term... it's the inflection point on the graph. I can't paste the graph because they've disabled copying on the test...

It's easier if you've read through the whole question, but thanks for the effort!!

What is the E that you refer to in E=0? Is that the potential?