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Offline avnis

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electrochemical cell question with unkown
« on: April 03, 2011, 10:38:28 PM »
The solutions at the two Pb electrodes of a concentration cell were prepared as follows:




Cell A: A mixture of 1.00 mL of 0.0500 M Pb(NO3)2 with 4.00 mL of 0.0500 M KX (the soluble potassium salt of an unspecified monovalent ion X-).



Some PbX2(s) precipitates.




Cell B: 5.00 mL of 0.0500 M Pb(NO3)2.

The cell potential was measured to be 0.06800 V at 25 °C.
By use of the Nernst equation, determine the concentration (M) of Pb2+ in the solution of Cell A.

Offline Borek

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Re: electrochemical cell question with unkown
« Reply #1 on: April 04, 2011, 04:53:01 AM »
You have to show your attempts at solving the question to receive help. This is a forum policy.

This is just a concentration cell, direct use of Nernst equation.
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Offline avnis

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Re: electrochemical cell question with unkown
« Reply #2 on: April 04, 2011, 01:56:03 PM »
You have to show your attempts at solving the question to receive help. This is a forum policy.

This is just a concentration cell, direct use of Nernst equation.


i am sorry i wasn't aware but, here is what i was thinking of doing i thought i would just pluging in the numbers but then i don't know the Ecell i mean the stand value, so i get stuck there.

Offline DevaDevil

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Re: electrochemical cell question with unkown
« Reply #3 on: April 04, 2011, 02:05:29 PM »
Can you start by writing down and balancing the chemical reaction(s) that take place?

What are the equations you need?

If you write all this down systematically, you will see which values you lack.



As for Ecell0, write down the equations taking place and you will see what it is.

Offline avnis

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Re: electrochemical cell question with unkown
« Reply #4 on: April 04, 2011, 02:16:14 PM »
Can you start by writing down and balancing the chemical reaction(s) that take place?

What are the equations you need?

If you write all this down systematically, you will see which values you lack.



As for Ecell0, write down the equations taking place and you will see what it is.

ohk so i thought it would be pb and k that will cause the reaction right, but a friend of mine says it is pb and x so i if it pb and k i can solve it( it is a piece of cake), but if it x i am doomed :(, i am unable to decide which one it is

Offline DevaDevil

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Re: electrochemical cell question with unkown
« Reply #5 on: April 04, 2011, 02:18:33 PM »
what equilibrium is there in the one half cell, and which in the other?

and no, neither X nor K is used in the reaction

Offline avnis

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Re: electrochemical cell question with unkown
« Reply #6 on: April 04, 2011, 02:22:31 PM »
what equilibrium is there in the one half cell, and which in the other?

and no, neither X nor K is used in the reaction

oh i haven't haven't thought of it like this

well in cell A it is a smaller concentration of pb ion and in cell B a larger so this cell will be driven by concentration difference so the electrons will flow from higher concentration to lower concentration

Offline avnis

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Re: electrochemical cell question with unkown
« Reply #7 on: April 04, 2011, 02:32:49 PM »
what equilibrium is there in the one half cell, and which in the other?

and no, neither X nor K is used in the reaction

oh i haven't haven't thought of it like this

well in cell A it is a smaller concentration of pb ion and in cell B a larger so this cell will be driven by concentration difference so the electrons will flow from higher concentration to lower concentration


oh now i am officially lost !!!!!!

Offline avnis

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Re: electrochemical cell question with unkown
« Reply #8 on: April 04, 2011, 02:36:26 PM »
what equilibrium is there in the one half cell, and which in the other?

and no, neither X nor K is used in the reaction

would it be lead ion and nitrate ions doing it

Offline DevaDevil

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Re: electrochemical cell question with unkown
« Reply #9 on: April 04, 2011, 02:56:26 PM »

well in cell A it is a smaller concentration of pb ion and in cell B a larger so this cell will be driven by concentration difference so the electrons will flow from higher concentration to lower concentration

indeed, so the only species participating in the reaction is lead.

Pb --> Pb2+ + 2 e- in one half

Pb2+ + 2 e- --> Pb in the other


what does this mean for Ecell0? (standard cell potential, where concentration is not factored in)

Offline avnis

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Re: electrochemical cell question with unkown
« Reply #10 on: April 04, 2011, 02:58:51 PM »

well in cell A it is a smaller concentration of pb ion and in cell B a larger so this cell will be driven by concentration difference so the electrons will flow from higher concentration to lower concentration

indeed, so the only species participating in the reaction is lead.

Pb --> Pb2+ + 2 e- in one half

Pb2+ + 2 e- --> Pb in the other


what does this mean for Ecell0? (standard cell potential, where concentration is not factored in)


it means that it is zero.

Offline avnis

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Re: electrochemical cell question with unkown
« Reply #11 on: April 04, 2011, 03:01:04 PM »

well in cell A it is a smaller concentration of pb ion and in cell B a larger so this cell will be driven by concentration difference so the electrons will flow from higher concentration to lower concentration

indeed, so the only species participating in the reaction is lead.

Pb --> Pb2+ + 2 e- in one half

Pb2+ + 2 e- --> Pb in the other


what does this mean for Ecell0? (standard cell potential, where concentration is not factored in)

it just meansEcell0 is zero so , now for he equation my Q will

Pb cell B/ Pb cellA

Offline DevaDevil

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Re: electrochemical cell question with unkown
« Reply #12 on: April 04, 2011, 03:02:19 PM »

it just meansEcell0 is zero

indeed. So try to solve it now.

Offline avnis

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Re: electrochemical cell question with unkown
« Reply #13 on: April 04, 2011, 03:18:02 PM »

it just meansEcell0 is zero

indeed. So try to solve it now.

OHK  got that now next part is saying

In Cell A, how many moles of X- have reacted with Pb2+?

i was doing thi converting the concentration of Pb into moles and that will be how any avaialbe to react and only that many will react but that isn't correct

Offline DevaDevil

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Re: electrochemical cell question with unkown
« Reply #14 on: April 04, 2011, 03:26:59 PM »
I assume they mean how many PbX2 has crystallized?

If your answer at 1) is correct, you have a concentration of Pb2+.

You then calculate how many moles of Pb2+ this is.
Your original number of moles of lead you can calculate from the lead nitrate solution you started with.

The difference is the number of moles that reacted


ps. use the numbers and formulas here if you want, then we can have a better look at where you might be going wrong

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