Strong and weak acids — answer key

1. MQ15 · Practice

Model answer. A strong acid is completely dissociated: every HI molecule splits into an H⁺ ion and an I⁻ ion, so the equation has a one-way arrow: HI(aq) → H⁺(aq) + I⁻(aq). The ⇌ arrow is for a weak acid, which is only partially dissociated.

A full answer includes

• HI(aq) → H⁺(aq) + I⁻(aq).

Also accept

• HI(aq) → H⁺(aq) + I⁻(aq)

Watch for

• “HI(aq) ⇌ H⁺(aq) + I⁻(aq).” — The ⇌ arrow means a reversible reaction in which many molecules stay whole: that is a weak acid.
• “HI(aq) → H⁻(aq) + I⁺(aq).” — The acid gives a hydrogen ion, H⁺, which is positive. The iodide ion is negative.
• “2HI(aq) → H₂(g) + I₂(aq).” — An acid in water splits into ions, not into elements: no gas forms.

2. MQ16 · Practice

Model answer. Hydrochloric acid is completely dissociated, so its equation has a one-way arrow: HCl(aq) → H⁺(aq) + Cl⁻(aq). Ethanoic acid is partially dissociated: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq). The ⇌ arrow means the reaction is reversible: ions join back into molecules all the time, so most molecules stay whole.

A full answer includes

• (1) →
• (2) ⇌
• (3) CH₃COO⁻(aq)
• (4) reversible
• (5) whole (undissociated)

Also accept

• →; ⇌; CH₃COO⁻(aq); reversible; whole

Watch for

• “A weak acid has no H⁺ ions.” — A weak acid is partially dissociated: a few of its molecules have split into H⁺ ions and negative ions, so its solution has some H⁺ ions, just fewer than a strong acid at the same concentration.
• “The ethanoate ion is CH₃COOH⁻.” — The molecule loses its H⁺, so the ion has one hydrogen fewer: CH₃COO⁻.

3. MQ17 · Practice

Options as numbered on the pupil sheet: 1 — proton acceptor; 2 — neither; 3 — proton donor.

Model answer. An acid is a proton donor: it gives H⁺ ions. A base is a proton acceptor. Sulfuric acid gives protons to the oxide ions, which become water, so H₂SO₄ is the donor and O²⁻ the acceptor. The magnesium ion neither gives nor takes a proton. HF gives a proton to NH₃, which becomes NH₄⁺: HF is the donor and NH₃ the acceptor.

A full answer includes

• 1. H₂SO₄: proton donor.
• 2. The oxide ion, O²⁻: proton acceptor.
• 3. The magnesium ion, Mg²⁺: neither.
• 4. HF: proton donor.
• 5. NH₃: proton acceptor.

Also accept

• donor; acceptor; neither; donor; acceptor

Watch for

• “NH₃ is the proton donor, because NH₄⁺ has more hydrogen.” — NH₄⁺ has more hydrogen because NH₃ took a proton: NH₃ is the acceptor.
• “Mg²⁺ accepts protons, because magnesium oxide is the base.” — In magnesium oxide it is the oxide ion, O²⁻, that takes the protons. Mg²⁺ is unchanged.

4. MQ18 · Practice

Model answer. Strong or weak tells you how completely the acid dissociates: hydrobromic acid is completely dissociated, propanoic acid only partially. Concentration tells you how much acid is dissolved in each dm³: P has 200 times as much. So Q is a dilute solution of a strong acid, and P a concentrated solution of a weak acid.

A full answer includes

• Q holds the strong acid, even though it is the more dilute solution.

Also accept

• Q is strong but dilute

Watch for

• “A strong acid is a concentrated acid.” — Strong describes how completely the acid dissociates into ions. Concentrated describes how much acid is dissolved in each dm³. A strong acid can be dilute, and a weak acid concentrated.
• “Both are strong acids, because every acid is strong.” — Acids differ: propanoic acid is only partially dissociated, so it is weak.
• “Hydrobromic acid is weak, because it is dilute.” — Dilute hydrobromic acid is still completely dissociated, so it is still a strong acid.

5. MQ19 · Practice

Model answer. (a) HF has the higher pH at the same concentration, so it is the weak acid. (b) HF(aq) ⇌ H⁺(aq) + F⁻(aq). (c) Hydrobromic acid is completely dissociated; hydrofluoric acid is only partially dissociated, so most of its molecules stay whole. Its solution has a lower concentration of H⁺ ions, so its pH is higher.

A full answer includes

• (a) Hydrofluoric acid, HF.
• (b) HF(aq) ⇌ H⁺(aq) + F⁻(aq).
• (c) HF is only partially dissociated.
• (c) So it has a lower concentration of H⁺ ions.
• (c) A lower concentration of H⁺ ions means a higher pH.

Also accept

• HF; HF(aq) ⇌ H⁺(aq) + F⁻(aq); partially dissociated, fewer H⁺ ions

Watch for

• “A strong acid is a concentrated acid.” — Strong describes how completely the acid dissociates into ions. Concentrated describes how much acid is dissolved in each dm³. A strong acid can be dilute, and a weak acid concentrated.
• “HF is more dilute.” — The two acids have the same concentration, 0.10 mol/dm³. The difference is how completely they dissociate.

6. MQ20 · Practice

Model answer. A weak acid is partially dissociated: a few citric acid molecules split into H⁺ ions and negative ions, and most stay whole. The H⁺ ions make lemon juice acidic and sour; there are few of them, which is one reason why it is safe to drink.

A full answer includes

• It has some H⁺ ions, but most of its molecules stay whole.

Also accept

• Some H⁺ ions; most molecules whole

Watch for

• “A weak acid has no H⁺ ions.” — A weak acid is partially dissociated: a few of its molecules have split into H⁺ ions and negative ions, so its solution has some H⁺ ions, just fewer than a strong acid at the same concentration.
• “All acids are dangerous.” — Some are: concentrated strong acids burn skin. But many acids are weak or very dilute, like the citric acid in lemon juice and the ethanoic acid in vinegar, and we eat them.
• “All its molecules have split into ions.” — That describes a strong acid, completely dissociated. A weak acid keeps most of its molecules whole.

7. MT3 · Apply

Model answer. Magnesium reacts with H⁺ ions to make hydrogen. U is hydrochloric acid, a strong acid, completely dissociated, so it has a high concentration of H⁺ ions and fizzes fast. V is ethanoic acid, a weak acid, only partially dissociated: at the same concentration it has far fewer H⁺ ions, so fewer meet the magnesium each second and it fizzes slowly.

A full answer includes

• (a) V.
• (b) V is only partially dissociated.
• (b) So V has a lower concentration of H⁺ ions than U.
• (b) Fewer H⁺ ions meet the magnesium each second, so it fizzes more slowly.

Also accept

• V; partially dissociated, fewer H⁺ ions

Watch for

• “A strong acid is a concentrated acid.” — Strong describes how completely the acid dissociates into ions. Concentrated describes how much acid is dissolved in each dm³. A strong acid can be dilute, and a weak acid concentrated.
• “A weak acid has no H⁺ ions.” — A weak acid is partially dissociated: a few of its molecules have split into H⁺ ions and negative ions, so its solution has some H⁺ ions, just fewer than a strong acid at the same concentration.