pH & pOH of Strong Acids & Bases (College Board AP® Chemistry): Study Guide
pH & pOH of Strong Acids & Bases
Strong acids
Most strong acids are monoprotic and fully ionize in aqueous solutions
The strong acids are HCl, HBr, HI, HClO4, HNO3 and H2SO4 (first ionization). Treat any other acid as weak (Ka < 1)
Due to the acid fully ionizing, there is a high concentration of H+/H3O+ ions resulting in the solution having a low pH
The H+/H3O+ concentration is equal to the molar concentration of the acid:
[H+ (aq)] = Mstrong acid
This can be used to calculate the pH using:
pH = −log10[H+ (aq)]
The position of the equilibrium is so far over to the right that you can represent the reaction as an irreversible reaction
The ionization of a strong acid
HA (aq) → H+ (aq) + A– (aq)
The single arrow shows an irreversible reaction
The hydrogen ions form from the complete ionization of the strong acid
Examiner Tips and Tricks
[H3O+] equals the acid's starting concentration only for the six strong acids.
For a weak acid such as HF, HNO2 or HCOOH, [H3O+] is much smaller than the starting concentration, so you need Ka and an ICE table
Strong bases
Strong bases, such as group I and group II hydroxides (e.g. NaOH, Ba(OH)2), completely dissociate in aqueous solution
A large concentration of OH– ions is present in the solution, giving it a high pH
Group I hydroxides produce one hydroxide ion per formula unit; group II hydroxides, such as Ca(OH)2, produce two
Therefore the concentration of hydroxide ions can be calculated using:
[OH–] = Mstrong base × number of OH– ions per formula unit
(1 for a group I hydroxide, 2 for a group II hydroxide)
This can then be used to calculate pOH:
pOH = −log [OH–]
pOH can then be used to calculate pH:
pH = 14.00 − pOH (at 25 °C)
The dissociation of a strong base can be represented using the equation below
The dissociation is complete, so it is shown with a single arrow (→)
The dissociation of a strong base
MOH (aq) → M+ (aq) + OH– (aq)
The single arrow shows an irreversible reaction
The hydroxide ions form from the complete dissociation of the strong base
Worked Example
Worked Example
Calculate the pH for a solution with a hydrogen ion concentration of 1.6 × 10-4 M.
Calculate the pH of a solution containing 0.00052 M Ba(OH)2.
Answers:
Answer 1:
The pH of the solution is:
pH = −log [H+]
pH = −log 1.6 × 10-4
pH = 3.80
Answer 2:
The pH of the solution is:
[OH-] = 0.00052 × 2 = 0.00104
pOH = −log 0.00104 = 2.98
pH = 14 − 2.98 = 11.02
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