[brandnewrocks13]

How many grams of sodium lactate (NaC3H5O3) should be added to 1.00 L of 0.140 M lactic acid  (C3H5O3) to form a buffer solution with pH= 4.10? Assume that no volume change occurs when the NaC3H5O3 is added. The Ka is 1.8E-5

I cannot get this problem to work out with the correct answer.

I set up the Ka=[H+][C3H5O3]/[HC3H5O3]  and solved for the [C3H5O3] and then converted it to moles.  what am I missing here?

[Borek]

Hard to say. Show details of your work.

[brandnewrocks13]

So i started with the equation HC3H5O3  H+ + C3H5O3-

I then set up the Ka expression by doing Ka= [H+][C3H5O3-]/[HC3H5O3]
So I then plugged the numbers and set the top of the expression as x^2 because there is a one to one ratio
1.8E-5=x^2/[.140M]
I solved for x and got .001587 M, and then converted it to grams using molar mass
so I got .14 g.

I didn't figure in the pH at all, so I'm wondering if I'm doing it way wrong.
Do I need to use the equation pKa=pH-log[base]/[acid]

I hope that is a little more clear of what i've done.

[Borek]

OK, that's not the way to do it.

Henderson-Hasselbalch equation in fact doesn't differ from the Ka definition - it is just solved for H⁺ and log is taken, so at first I thought you can be OK. But what you have done is you calculated what will happen if the acid dissociates. But you don't have just an acid solution, you have solution that already contains acid and conjugate base., so you can't treat it just as if it was acid dissociation.

Solve Ka definition for concentration of lactate - you will have on the right only known values of [H⁺], Ka and [HLactate] (HLactate being placeholder for lactic acid). Just plug the numbers.

[brandnewrocks13]

ok.. so let me see if I'm understanding,

I set it up like Lactate=Ka[lactic acid]/ [H+]
i did that and got a  concentration of .0317 M

for the H+, can I just take the pH of 4.10 and do 10^-4.10 to get the H+ concentration?
I then converted .0317 M to moles of Lactate and got 2.82 g..  does that seem right?

[brandnewrocks13]

I just entered that answer into my online hw and it still says that it isn't right (2.8 g)

what am i doing wrong?

[Borek]

Try with molar mass of the sodium lactate, not lactic acid