Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: _Bd_ on October 17, 2010, 03:45:54 AM
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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!
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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...