Strengths of Acids

1

• Strengths of Acids Bases

• Acids

• Strong acids dissociate completely in solution.

eg
HCl(aq) ? H(aq) Cl-(aq)
or
HCl(aq) H2O(l) ? H3O(aq)
Cl-(aq)
HCl H H3O

• Weak acids do not dissociate completely (they
  reach equilibrium.)

eg
HCN ? H3O
HCN ? H3O
A 1.0M solution of HCl has a H3O of 1.0M,
however a 1.0M solution of HCN will have a H3O
lt 1.0M.
eg
2

• Bases

• Strong bases dissociate or react completely.

eg
NaOH(aq) ? Na(aq) OH-(aq)

• The concentration of NaOH dissolved equals OH-.

Note all of the alkali metal hydroxides are
strong bases. (NaOH, KOH, LiOH, etc)

• Weak bases do not dissociate or react completely.

eg

• Not all of the NH3 will react, therefore the
  amount of NH4 produced is not the same as the
  amount of NH3 initially

NH4 lt NH3
3

• Weak Acid Equilibrium

eg
HCN is a weak acid
KA
or
KA

• The greater the KA the stronger the acid (ie
  the higher the H3O)

• KA values are measured at 25?C (see data booklet.)

• Changing temperature will change KA.

4
What is the pH of 0.25 mol of hydrocyanic acid
(HCN) dissolved in 1.0L of solution? KA for HCN
is 6.2 x 10-10
eg
I
0.25 M
0
0
C
- x
x
x
E
0.25 - x
x
x
Assuming x is negligible compared to 0.25
KA
6.2 x 10-10

x 1.2 x 10-5 M
H
pH -log (1.2x10-5)
pH 4.90
(if this had been HCl the pH would have been 0.60)
5
Calculate KA for carbonic acid (H2CO3) if a 0.24M
solution of the acid has a pH of 3.49.
eg
H 3.2 x 10-4 M
pH 3.49
I
0.24 M
0
0
C
- 3.2x10-4
3.2x10-4
3.2x10-4
E
0.24
3.2x10-4
3.2x10-4
KA
KA
4.3 x 10-7
6
Note Polyprotic acids have a different KA for
each H.
eg
KA1
KA2
KA3
And KA1 gtgt KA2 gtgt KA3