# Homework Help: Buffers, pH, and strong base

1. Oct 27, 2008

### needphyshelp

1. The problem statement, all variables and given/known data
Calculate the pH after 0.030 mol NaOH is added to 1.00 L 0.100 M HC3H5O2 and 0.100 M NaC3H5O2

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

3. 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 Kaeq1=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...

2. Oct 27, 2008

### Staff: Mentor

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.

3. Oct 27, 2008

### 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?

4. Oct 27, 2008

### Staff: Mentor

Yes.

5. Oct 27, 2008

oh... duh..
thanks :-)