Question

Calculate the pH of each of the following solutions.
(a) 0.300 M propanoic acid (HC3H5O2, Ka = 1.3 10-5)
(b) 0.300 M sodium propanoate (NaC3H5O2)
(c) pure H2O
(d) 0.300 M HC3H5O2 and 0.300 M NaC3H5O2

Answer 1

a)Let HA = the weak acid, proanoic acid.
..............HA ==> H^+ + A^-
initial......0.300....0......0
change.........-x......x......x
equil........0.300-x...x.......x

Ka = (H^+)(A^-)/(HA)
Substitute from the ICE chart and solve for x, then convert to pH.

b)
This is a salt and the pH will be determined by the hydrolysis of the salt.
A^- + HOH ==> HA + OH^-
Set up an ICE chart similar to the one in (a) and solve for OH^-, then convert to pH.

(c) Pure water ionizes as
HOH => H^+ + OH^-
Set up an ICE chart and use
Kw = (H^+)(OH^-), solve for H^+ and convert to pH.

(d)
This is a buffered solution. Use the Henderson-Hasselbalch equation.
Post your work if you get stuck.