[supaflyz]

A buffer that contains 0.609 M of acid, HA and 0.636 M of its conjugate base A-, has a pH of 3.93. What is the pH after 0.0453 mol of NaOH are added to 0.838 L of the solution.

Hey guys can you help me on this problem.  I was going over my homework before my exam this Wednesday and got stuck on this problem.  I didn't know how to do it so I stumbled across this forum.  Can you guys help me on this problem.  The answer is 4.01 but  I dont know how to get it. 

[Borek]

Start with Henderson-Hasselbalch equation. Calculate pKa. Then assume the neutralization reaction went to completion.

[supaflyz]

ok so the equation is ph=pka+log {base/acid).  So the mols and liters they gave me are not important? 

[Borek]

You will need them for neutralization stoichiometry.

[supaflyz]

Hey Borek I just try it so from the equation pH=pka+log (base/acid) I put in

3.93=pka+log(0.636/0.609)   so I got 3.911=pka.  I need the ka values right?  So I would

use 3^3.911 or -log(3.911.  I just never done this type of problem before.

[leve]

For neutralization, the equilibrium lies far to the right so there is no need to calculate the concentrations of acid base. Use moles and stoichiometry.

[Borek]

so I got 3.911=pka.

That's correct. You will not need Ka value. Now just assume that neutralization went to completion and calculate new concentrations of A^- and HA, this is a simple stoichiometry.