Calculate the pH of the solution.?

Question

How do I calculate the pH of a solution containing .80 M lactic acid (HC3H5O3) and .20 M sodium lactate (NaC3H5O3) where lactic acid has a Ka value of 1.4 x 10^-4(ten to the negative four)?
Thank you!

Answer 1

So you need to calculate out the concentration of H+ ion, before you may calculate pH value based on this:
pH = - log ([H+]).
Let us do it step by step. First, write out the reaction equation and balence it:
HC3H5O3 <==> H(+) + C3H5O3(-)
Second, write out the representation of Ka:
Ka = 1.4x10^-4 = [H+]*[C3H5O3-] /[HC3H5O3]
= [H+]*(0.20M/0.80M) = [H+]/4
since NaC3H5O3 completely dissociate to Na+ and C3H5O3- in the solution. Therefore:
[H+] = 4*Ka and:
pH = -log (4*Ka) = 3.3