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J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry 4th edition

Problem 15.81

A solution is 0.405 M HCOOH (formic acid) and 0.326 M in the salt HCOONa (sodium formate). What is the pH of the buffer solution?






Answer

Write the dissolution reaction for sodium formate, the acid–base equilibrium for formic acid, write the mass action expression for Ka, set up a table of concentrations, and solve.

HCOONa(aq)
Na+(aq)
+
HCOO(aq)
HCOOH(aq)
+
H2O(l)
H3O+(aq)
+
HCOO(aq)
Initial
0.405
0
0.326
Change
– x
+ x
+ x
Equilibrium
0.405 – x
x
0.326 + x

Approximate? 0.405/1.8×10–4 = 2000 > 100, yes; 0.362/1.8×10–4 = 2000 > 100, also yes.

x = [H3O+] = 2.2×10–4 M

pH = –log[H3O+] = –log[2.2×10–4] = 3.66