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Topic: Buffers  (Read 1852 times)

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Offline _Bd_

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Buffers
« on: October 17, 2010, 03:45:54 AM »
Im trying to do a buffer problem from the book but I dont get the answer in the back. I already looked and re read everything and Im doing everything accordingly but I just dont get the answer!
problem:
Find the pH of a buffer solution containing .15M HCHO_2  and .1 M of NaCHO_2

Attempt at solution:

-------HCHO_2 + H_2O <--> NaCHO_2 + H_3O ---
Ini.. .15 .... .1 ...0
Chang ... -x .. +x ..+x
Equil.... (.15 - x)..(.1 + x) ... x

so knowing that K= [H3O][A-] / [HA] and that K = 1.8 x 10^-5
i substitute in values and since .1 is >> than 1.8 x10^-5 then I can ignore any x's being added or subtracted
giving the following

x/(.15) = 1.8x10^-5
so then the value I get for ex is 2.7 x 10 ^-5 and therefore the pH should be the -log of that (which is 4.56) but the back of the book says its 3.57

plz help its driving me nuts!

Offline JustinCh3m

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Re: Buffers
« Reply #1 on: October 18, 2010, 07:20:42 PM »
you wrote that you had gotten to this point:  x/(.15) = 1.8x10^-5

I think you need to take a look at the numerator "x" -- you've forgotten something there.  take a look at sodium acetate product and what's left after you do the "5% approximation".

Also, that's the Ka of acetic acid.  Did you mean to write out HOAc in your equation?  Just wonderin...

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