Calculating pH of a Buffer Solution with Added Strong Base

[needphyshelp]

Homework Statement

Calculate the pH after 0.030 mol NaOH is added to 1.00 L 0.100 M HC3H5O2 and 0.100 M NaC3H5O2

Homework Equations

Ka=[H+][A-]/[HA]
pH=-log([H+]) or 14-pOH=ph

The Attempt at a Solution

HC3H5O2 <-> H⁺ + C3H5O2^-
NaC3H5O2 <-> Na⁺ + C3H5O2^-

I know that Na+ has no acid/base properties, so I think that this equation is unimportant and I will be focusing on the first equation. I also know Ka_eq1=1.3x10^-5

I tried to set up an equilibrium equation, but I'm not sure if I should be completely ignoring the sodium equation.

1.3x10^-5=(x²)/(.100-.030)
the numerator= [H+][C3H5O2-]

Any input would be appreciated...

 

[Borek]

pH of buffer depends only on pKa and the ratio of acid and conjugate base - as described by Henderson-Hasselbalch equation. Use simple stoichiometry to calculate new amounts of both acid and base after NaOH was added - and you are done.


&& Buffer Maker - the ultimate buffer calculator

 

[needphyshelp]

Wouldn't that yield
pH=log(1.3e-5)+(.030/.1-.030) = 4.52? This answer is incorrect. Is there an initial concentration for C3H5O2- that I am missing?

 

[Borek]

0.100 M NaC3H5O2

Is there an initial concentration for C3H5O2- that I am missing?

Yes.

 

[needphyshelp]

oh... duh..
thanks :-)