# Ph of buffer solution

1. Apr 14, 2007

### lizzyb

Question

A buffer solution is prepared by mixing 380.0 ml of 0.250 M propionic acid, C2H5COOH, and 120.0 ml of 0.350 M sodium propionate, C2H5COONa. Calculate the pH of the prepared buffer solution. For propionic acid, Ka = 1.34 X 10^-5.

Work thus far

Code (Text):

We have this

C2H5COOH <--> C2H5COO- + H+

Since the C2H5COONa goes to completion and is left
with C2H5COO-, we can determine the initial amount
of C2H5COO-, and then we use the x-method to determine
the pH.

Initial Molarities:

.250 M .380 L     .250 * .380
[C2H5COOH] = --------------- = ------------ = .19 M
(.380 + .120)L        .5

.350 M * .12 L     .350 * .12
[C2H5COO-] = ---------------- = ------------ = .084 M
(.380 + .120) L      .5

C2H5COOH   <-->     C2H5COO-    +        H+

.19                 .084               0       initial
-x                  x                 x       change
.19-x               .084+x              x       equilibrium

so,

(.084 + x) x
Ka = 1.34 X 10^-5 = --------------
(.19-x)

==> x = 3.029 X 10^-5, pH = -log(x) = 3.002

does this look okay to you?

2. Apr 14, 2007

### symbolipoint

Yes. ..... ..

3. Apr 14, 2007

### lizzyb

thank you! :-)