pH and pOH of Strong Acids and Bases

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: $\text{pH} = -\log [\text{H}^+]$

• pOH Calculation: $\text{pOH} = -\log [\text{OH}^-]$

• Relationship between pH and pOH: $\text{pH} + \text{pOH} = 14$

• Ion Product of Water: $[\text{H}^+][\text{OH}^-] = 1.0 \times 10^{-14}$

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: $[\text{OH}^-] = \text{molarity} \times \text{number of OH}^-$

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:

• $[\text{OH}^-] = 0.308 \times 2 = 0.616\ \text{M}$

• $\text{pOH} = -\log(0.616) = 0.210$

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

Type	Given	Find	Equation
Strong Acid	[H+]	pH	$\text{pH} = -\log [\text{H}^+]$
Strong Base	[OH-]	pOH	$\text{pOH} = -\log [\text{OH}^-]$
Relationship	pH or pOH	pOH or pH	$\text{pH} + \text{pOH} = 14$

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.