Strong Acids and Bases; pH and pOH
Name one strong acid and one strong base.
HCl (strong acid), NaOH (strong base)
What makes an acid or base “weak”?
It only partially ionizes in solution.
What is the product of a reaction between HCl and NaOH?
NaCl and H₂O
What is a buffer?
A solution that resists changes in pH when small amounts of acid or base are added.
What is the Henderson-Hasselbalch equation?
pH = pKa + log([A⁻]/[HA])
What is the pH of a 0.01 M HCl solution?
pH = 2
What does a small Ka value say about an acid?
It is a weak acid and does not dissociate much.
What’s the term for the point where moles of acid = moles of base in a titration?
Equivalence point
Name a conjugate acid base pair in the equation:
HCl + NH3 -> NH4+ + Cl-
HCl and Cl-
If [A⁻] = [HA], what is the pH also equal to?
pH = pKa
What is the pOH of a 0.001 M NaOH solution?
pOH = 3
Write the equilibrium expression for acetic acid.
CH₃COOH (aq) ⇌ H⁺ (aq) + CH₃COO⁻ (aq)
Ka = [H⁺][CH₃COO⁻] / [CH₃COOH]
Why does phenolphthalein (a chemical indicator) change color in a titration?
It responds to changes in pH near the equivalence point.
What happens to the pH of a buffer when a small amount of HCl is added?
The pH doesn't change significantly
A buffer has [A⁻] = 0.3 M and [HA] = 0.2 M with pKa = 4.8. What is the pH?
pH ≈ 4.98
If the pH of a solution is 11, what is the [OH⁻]?
[OH⁻] = 10⁻³ M
If the pKa of a weak acid is 4.7, is it stronger or weaker than an acid with a pKa of 3.5?
Weaker (higher pKa = weaker acid)
What is the pH at the equivalence point in a strong acid–strong base titration?
pH = 7
Why can’t a buffer be made from a strong acid and its conjugate base?
Strong acids fully dissociate, so their conjugate base is too weak to act as a base.
How does pH affect the solubility of a weakly basic salt like Mg(OH)₂?
Lower pH increases solubility (more H⁺ drives reaction forward)
Calculate the pH of a 0.005 M Ba(OH)₂ solution.
[OH⁻] = 0.01 M → pOH = 2 → pH = 12
Calculate the pH of a 0.1 M HF solution with Ka = 6.6 × 10⁻⁴.
H ≈ 2.1 (requires RICE table and solving for [H⁺])
A 25 mL solution of 0.1 M HNO₃ is titrated with 0.2 M NaOH. How many mL of base are needed to reach the equivalence point?
12.5 mL
A buffer is made with 0.25 M NH₃ and 0.20 M NH₄Cl. Calculate the pH (Kb for NH₃ = 1.8 × 10⁻⁵).
pH ≈ 9.3 (requires Henderson-Hasselbalch or RICE and pOH calculation)
A saturated solution of CaF₂ has a Ksp of 3.9 × 10⁻¹¹. Find the solubility of CaF₂ in pure water.
Solubility ≈ 2.1 × 10⁻⁴ M