pH and pOH of Strong Acids and Bases

Strong Acids and Bases in Aqueous Solution

Strong acids and strong bases completely ionize in water, allowing for straightforward calculation of hydrogen ion [H+] and hydroxide ion [OH-] concentrations. These concentrations are directly related to the acid or base molarity.

• Strong Acid: A substance that completely dissociates in water to produce H+ ions.

• Strong Base: A substance that completely dissociates in water to produce OH- ions.

Key Equations

• pH Calculation:

• pOH Calculation:

• Relationship between pH and pOH:

• Ion Product of Water:

Ions of Strong Bases

Strong bases may contain the following ions:

• OH- (hydroxide ion)

• O2- (oxide ion)

• NH2- (amide ion)

Calculation of pH or pOH for Strong Bases

• For bases that release more than one OH- per formula unit, multiply the molarity by the number of OH- ions released.

• Use the formula:

Example Problem

Example: If the concentration of Ba(OH)2 solution is 0.308 M, calculate its pOH.

• Ba(OH)2 dissociates to give 2 OH- per formula unit:

Practice Problems

1. An aqueous solution of HClO4 has a pH of 3.44. Find the molar concentration of HClO4 solution. Answer: 3.63 × 10-4 M

2. Calculate the pH of a 2.15 × 10-3 M LiOH solution. Answer: 11.33

3. HBr is a strong acid (pKa ≈ -9). Calculate pH, [OH-], and pOH of a 7.1 × 10-4 M HBr solution. Answer: 3.15

Summary Table: Calculations for Strong Acids and Bases

Additional info: For polyprotic acids or bases that release more than one H+ or OH- per formula unit, multiply the molarity by the number of ions released before calculating pH or pOH.