[Kuahji]

Just doing an online practice quiz.

A solution is 0.100 M in acetic acid and 0.100 M in sodium acetate.  What is the concentration of H+?

So you have the equation
HC2H3O2 + H2O -> H3O+ + C2H3O2-

I then setup the ICE table & got the equation

x^2/(.1-x) = 1.85 x 10^-5 (ka of acetic acid)

The equation works out to be x^2 + 1.85 x 10^-5 - 1.6 x 10^-6
x=.00133 which is the concentration of the H+

But clearly I must be doing something wrong, the answer is suppose to be 1.85x10^-5.  Where am I going wrong?

[Borek]

This is buffer solution.

[vhpk]

Means that you must use the Henderson-Hasselbalch equation:
pH = pKa + lg[A^-]/[HA]
A^-: the conjugated part

[Kuahji]

Means that you must use the Henderson-Hasselbalch equation:
pH = pKa + lg[A^-]/[HA]
A^-: the conjugated part

Thank you for pointing me in the right direction.   

[Kuahji]

Ok, so if anyone reads these things just for fun, here is the correct way to solve the problem.   

Start out with the equation & then make your ICE table
CH3CO2H + H2O -> H3O+ + CH3CO2
I .1                          0          .1
C -x                         x           x
E .1-x                       x          .1+x

So your equation then becomes

(x(.1+x))/(.1-x)=1.85 x 10^-5

Then its basic math to solve for x which gives the correct answer of 1.85 x 10^-5 M H+. 

[Borek]

No need for ICE table, you were told explicitely to use HH equation.