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Topic: equilibrium - is charge / mass balance Ok here?  (Read 3110 times)

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Offline zephyrblows

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equilibrium - is charge / mass balance Ok here?
« on: February 20, 2007, 10:10:45 PM »
Here is the question:

Given a solution containing 0.1M H2CO3 and 0.005M Pb2+
Also, we already know there is no PbCO3 precipitation
Find the minimum value of [H+]
----------------------
Ka1 of H2CO3 = 4.4 * 10-7
Ka2 of H2CO3 = 4.7 * 10-11
Ksp of PbCO3 = 1.5 * 10-15
----------------------
Here are my questions:
1.If not given, how do we know the extreme value of [H+] is a minimun, not a maximum?

2.I asked myself an equivalent (is it really equivalent??) problem:
Find the value of [H+] in a 0.1M H2CO3 solution in which [CO32-] is fixed at 3 * 10-13M
However, if [CO32-] is fixed, are charge and mass balance available?

Thanks a lot for any *delete me*!

Offline AWK

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Re: equilibrium - is charge / mass balance Ok here?
« Reply #1 on: February 21, 2007, 01:27:34 AM »
1. More basic solution - more CO32-. Hence we talk about minum pH at which PbCO3 start precipitate.
2. Ka1Ka2=[H+]2[CO32-]/[H2CO3]
AWK

Offline zephyrblows

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Re: equilibrium - is charge / mass balance Ok here?
« Reply #2 on: February 21, 2007, 03:28:12 AM »
Ka1Ka2=[H+]2[CO32-]/[H2CO3]
Well, I was trying my best avoiding this approach...but I think I should surrender :P

--------------------------------------BELOW IS MAINLY FOR BOREK (or other general approach lovers)-----------------------------------
Anyway, if I insist.... I guess I could try another perspective...
"It's not [CO32-] but [H+] that is fixed; the known [CO32-] is achieve by fixed [H+]."
Then I guess mass balance is available...
Is this correct? (My answer almost equals the one got from the net equation approach...)
« Last Edit: February 21, 2007, 04:29:30 AM by zephyrblows »

Offline AWK

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Re: equilibrium - is charge / mass balance Ok here?
« Reply #3 on: February 21, 2007, 04:25:48 AM »
Quote
I see! Thank you! But I guess you mean "we talk about minum [H+]," right?
I mean a minimum pH at which a precipitation PbCO3 starts. This is also a highest [H+] concentration
for the precipitation.
AWK

Offline zephyrblows

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Re: equilibrium - is charge / mass balance Ok here?
« Reply #4 on: February 21, 2007, 04:30:55 AM »
Oh, my bad, sorry for that.

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