8.2-8.3 pH and pOH of Strong and Weak Acids and Bases

8.2 pH and pOH of Strong Acids and Bases

AP Chemistry: Unit 8 Acids and Bases

Learning Objective:Calculate pH and pOH based on concentrations of all
species in a solution of a strong acid or a strong base.

Strong Acids

●Strong acids dissociate 100% in water.

●When acids dissociate only one H+ is dissociated
at a time.

●The [H3O+] after dissociation is equal to the
original acid molarity.

Example

Strong Base

●Strong bases dissociate 100% in water.

●The strong bases you need to know are Group 1
and Group 2 hydroxides

●For Group 1 bases the [OH-] after dissociation is
equal to the original base molarity.

●For Group 2 bases the [OH-] after dissociation is
equal to twice the original base molarity.

Example

Possible Quiz Questions

Multiple Choice

Which of the following pairs of mathematical expressions can be used to correctly calculate the pH and pOH of a 0.0015M KOH(aq) solution at 25 C?

Multiple Choice

Which of the following statements about the pH of 0.010 M HClO4 is correct?

Possible Quiz Questions

Multiple Choice

The equilibrium reaction shown above represents the partial ionization of the weak acid HCN. A 0.200M HCN solution has a pH around 4.95. If 0.049g (0.010 mol) of NaCN (s) is added to 100mL of 0.200 M HCN (aq), which of the following explains how and why the pH of the solution changes?

• The pH will be higher than 4.95 because adding CN- will disrupt the equilibrium resulting in an increased production of HCN that decreases the concentration of H3O+

• The pH will be less than 4.95 because adding CN- will disrupt the equilibrium, resulting in an increased production of HCN that decreases the concentration of H3O+

• The pH will be higher than 4.95 because CN- is a strong base that can neutralize HCN

• The pH will remain close to 4.95 because the K is so small that hardly any products form.

Multiple Choice

The equilibrium between a weak base and water is shown. The table shows the pH of three solutions at 25 C. Based on the information given, which of the following is true.

• Solutions with higher concentrations of (CH₃)NH have a higher pOH because to reach equilibrium a smaller amount of the conjugate acid (CH₃)NH₂⁺ is produced

• Solutions with higher concentrations of (CH₃)NH have a higher pOH because to reach equilibrium more OH^- is produced

• Solutions with higher concentrations of (CH₃)NH have a higher pH because to reach equilibrium a smaller amount of the conjugate acid (CH₃)NH₂⁺ is produced.

• Solutions with a higher concentration of (CH₃)NH have a higher pH because to reach equilibrium more OH^- is produced.