[fyodor.pav]

Hello everyone!  I'm currently reading Lehninger's Biochemistry 4th edition (for fun, not school), and I am stuck on the following problem...

Chapter 2 #13. Working with Buffers A buffer contains 0.010 mol of
lactic acid (pKa = 3.86) and 0.050 mol of sodium lactate per
liter. (a) Calculate the pH of the buffer. (b) Calculate the
change in pH when 5 mL of 0.5 M HCl is added to 1 L of the
buffer. (c) What pH change would you expect if you added
the same quantity of HCl to 1 L of pure water?

"A" I believe I can do easily, via pH = 3.86 + log(.05/.01) = 4.56

"B" is where I am having the problems.  Conceptually, I can see the reaction going like HCl  completely ionizes which raises H+ concentration, which effects the lactic acid/lactate balancing act.  My question is, how much of the H+ is absorbed by the lactate.

I tried looking up similar buffer problems on the net, and found http://zimmer.csufresno.edu/~davidz/Chem102/Gallery/Buffer/Buffer.html

Which calculates the moles of H+ added and would add that exact amount to the moles of lactic acid, while subtracting that exact amount from the moles of lactate, then recalculating concentrations and using pH = pKa + log(...) etc.  So in my mind that means the buffer absorbed the entire addition of H+ from the HCl.  Is this 100% correct?  Or is it true that the buffer really did NOT absorb the entire addition of H+ and this method for calculating the new pH just assumes it is close enough?

I hope I am making myself clear, it is a confusing question (for me atleast).

Thanks for any help.

[AWK]

_{"B" is where I am having the problems.  Conceptually, I can see the reaction going like HCl  completely ionizes which raises H+ concentration, which effects the lactic acid/lactate balancing act.  My question is, how much of the H+ is absorbed by the lactate.}
Before calculations do a simple stoichiometry -you have a strong acid and a salt of a weak acid. Assume that reaction proceeds completely, and neglect change of volume.
This is done in your link.

[Borek]

So in my mind that means the buffer absorbed the entire addition of H+ from the HCl.  Is this 100% correct?  Or is it true that the buffer really did NOT absorb the entire addition of H+ and this method for calculating the new pH just assumes it is close enough?

If they are still using Henderson-Hasselbalch equation that means they assumed added strong acid was a limiting reagent. In the case of excess HCl you can in most cases safely ignore buffer presence.