Calculate the ph of a mixture?

Question

Calculate the pH of a mixture containing equal volumes of
0.340 M HC3H5O2 ( use Ka = 1.30 x 10 -5 ) and 0.340 M NaC3H5O2 .

Answer 1

Use the Henderson-Hasselbach equation.

pH = pKa + log [base]/[acid]

Since [base]=[acid], the log term is 0 and pH = pKa, which is 4.89.

Answer 2

Henderson Hasslebach is the way to go. Keep in mind which is your acid and which is the conjugate base. Most often an alkali metal is used to get the conjugate base into the solution.

Answer 3

HC3H5O2<> H+ + C3H5O2-
Initial: .340M <-> 0 + .340M (from the NaC3H5O2)
Change: -x <-> +x + +x
_____________________________________________
Equilibrium: .340-x<->x + .340+x

Ka=concentration of products/reactants
1.3e-5=x(.340+x)/(.340-x)
solve for x
x=1.3e-5

pH=-log([H+])
pH=4.89