Salts. Since acid + base salt + H 2 O, when a salt is mixed in water, it dissociates. The cation and anion may be able to absorb H + and OH - ions. This could alter the pH of a solution. To determine if a salt solution is acidic or basic Divide the salt into cation and anion
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For weak acids and bases, we look at Ka and Kb of each. The higher value determines if the solution is acidic or basic.
HCl HBr HI HNO3 HClO3 HClO4 H2SO4
Group I hydroxides Heavy group 2 hydroxides
Example – Determine if the following solutions should be acidic or basic.
a. CaCl2 b. NaNO3 c. Na2SO3 d. KC2H3O2 e. NH4Br
Examples – Write out the important equilibrium that occurs when we add the following salts to water
a. NaClO b. NH4Cl c. KNO2
HClO + OH-
H3O+ + NH3
HNO2 + OH-
If Ka HF = 6.8 x 10-4 and Ka of NH4+ is 5.6 x 10-11, determine the pH of
a. 0.15 M NH4Cl solution b. 0.50 M NaF solution
a. Since NH4Cl will be acidic, treat NH4+ as a weak acid, ignore Cl-.
NH4+ NH3 + H+
Ka = [NH3][H+]
5.6 x 10-11 = x2
X = 2.90x 10-6
pH = -log 2.90x 10-6
NaF Na+ + F-
ignoretreat as a weak base
F- + H2O HF + OH-
We need Kb for F-, so alter the Ka
Ka x Kb = Kw
6.8 x 10-4 x Kb = 1 x 10-14
Kb = 1.47 x 10-11
Kb = [HF][OH-]
1.47 x 10-11 = x2
X = 2.71x 10-6
pOH = -log 2.71x 10-6
pH = 8.44