Acid base calculations

Question 1

How do you calculate the pH of a strong acid?

Answer 1

Use pH = -log[H⁺]. For a strong acid, [H⁺] = concentration of the acid (since it fully dissociates).

Example: Find the pH of 0.1 mol dm⁻³ HCl. pH = -log(0.1) = 1.00

Question 2

How do you calculate the pH of a strong base?

Answer 2

Find [OH⁻] (equal to the base concentration). Use Kw = [H⁺][OH⁻] to find [H⁺] (Kw = 1.00 × 10⁻¹⁴ at 25°C). Calculate pH = -log[H⁺].

Example: Find the pH of 0.02 mol dm⁻³ NaOH. [OH⁻] = 0.02 mol dm⁻³, [H⁺] = (1.00 × 10⁻¹⁴) ÷ (0.02) = 5.00 × 10⁻¹³, pH = -log(5.00 × 10⁻¹³) = 12.30

Question 3

How do you calculate the pH of a weak acid?

Answer 3

Use Ka = [H⁺]² / [HA] (assuming [H⁺] = [A⁻]). Solve for [H⁺]. Use pH = -log[H⁺].

Example: Find the pH of 0.10 mol dm⁻³ ethanoic acid (Ka = 1.75 × 10⁻⁵ mol dm⁻³). [H⁺] = √(Ka × [HA]) = √(1.75 × 10⁻⁵ × 0.10) = 1.32 × 10⁻³, pH = -log(1.32 × 10⁻³) = 2.88

Question 4

How do you calculate the pH of a weak base?

Answer 4

Use Kb = [OH⁻]² / [B] (assuming [OH⁻] = [BH⁺]). Find [OH⁻] and use Kw = [H⁺][OH⁻] to calculate [H⁺]. Use pH = -log[H⁺].

Question 5

How do you convert between Ka and pKa?

Answer 5

pKa = -log(Ka). Ka = 10⁻ᵖᴷᵃ.

Example: If Ka = 1.75 × 10⁻⁵, then pKa = -log(1.75 × 10⁻⁵) = 4.76.

Question 6

How do you calculate the pH of a buffer solution?

Answer 6

Use the Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]).

Example: A buffer contains 0.2 mol dm⁻³ ethanoic acid and 0.1 mol dm⁻³ sodium ethanoate (Ka = 1.75 × 10⁻⁵). pKa = 4.76, pH = 4.76 + log(0.1/0.2) = 4.76 - 0.301 = 4.46

Question 7

How do you calculate the pH at different points in a strong acid-strong base titration?

Answer 7

Before equivalence: Find excess H⁺ and use pH = -log[H⁺]. At equivalence: pH = 7 (only water and salt remain). After equivalence: Find excess OH⁻, use Kw to get [H⁺], then calculate pH.

Question 8

How do you calculate the pH at different points in a weak acid-strong base titration?

Answer 8

Before equivalence: Use Ka equation for weak acid. Half-neutralization point: pH = pKa. At equivalence: Use conjugate base hydrolysis to find pH (pH > 7). After equivalence: Use excess OH⁻ to find pH.

Question 9

How do you calculate the pH when mixing a strong acid and a strong base?

Answer 9

Find moles of H⁺ and OH⁻. Determine which is in excess. Find the new concentration after dilution. Use pH = -log[H⁺] (if acidic) or pOH = -log[OH⁻] then pH = 14 - pOH (if basic).

Example: Mix 50 cm³ of 0.1 M HCl with 25 cm³ of 0.1 M NaOH. Moles H⁺ = 0.1 × 0.050 = 0.005, Moles OH⁻ = 0.1 × 0.025 = 0.0025, Excess H⁺ = 0.005 - 0.0025 = 0.0025, New concentration = 0.0025 / 0.075 = 0.0333 mol dm⁻³, pH = -log(0.0333) = 1.48