Pupil sheet · Acids, bases and indicators

Strong and weak acids — questions

Name ______________________ Date ____________

Try every question before you use a nudge. Write in full sentences where you are asked to explain.

1. identify

Supplement. Hydriodic acid, HI, is a strong acid. Which equation shows what happens when it dissolves in water?

Circle one answer.

HI(aq) ⇌ H⁺(aq) + I⁻(aq)
2HI(aq) → H₂(g) + I₂(aq)
HI(aq) → H⁻(aq) + I⁺(aq)
HI(aq) → H⁺(aq) + I⁻(aq)

Nudge (only if you are stuck): A strong acid is completely dissociated. Which arrow shows a reaction that goes all the way?

2. state

Supplement. Fill in the gaps. Hydrochloric acid is a strong acid: HCl(aq) (1) ___ H⁺(aq) + Cl⁻(aq). Ethanoic acid is a weak acid: CH₃COOH(aq) (2) ___ H⁺(aq) + (3) ___. The arrow in the second equation means the reaction is (4) ___, so most ethanoic acid molecules stay (5) ___.

(1)

(2)

(3)

(4)

(5)

Nudge (only if you are stuck): A strong acid splits completely; a weak acid only partly.

3. identify

Supplement. Two reactions: H₂SO₄ + MgO → MgSO₄ + H₂O, and HF + NH₃ → NH₄⁺ + F⁻. Magnesium oxide is made of Mg²⁺ and O²⁻ ions. Match each particle to its role.

Write the number of the matching option.

proton acceptor
neither
proton donor

1. H₂SO₄: ______
2. The oxide ion, O²⁻: ______
3. The magnesium ion, Mg²⁺: ______
4. HF: ______
5. NH₃: ______

Nudge (only if you are stuck): A proton is an H⁺ ion. Which particle gives H⁺, and which takes it?

4. compare

Supplement. Bottle P holds propanoic acid, a weak acid, at 2.0 mol/dm³. Bottle Q holds hydrobromic acid, a strong acid, at 0.010 mol/dm³. Which statement is correct?

Circle one answer.

Both are strong acids, because every acid is strong.
Q holds a weak acid, because it is dilute.
Q holds the strong acid, even though it is the more dilute solution.
P holds the stronger acid, because it is more concentrated.

Nudge (only if you are stuck): Strong and concentrated describe two different things.

5. identify / give / explain

Supplement. Two acids have the same concentration, 0.10 mol/dm³. Hydrobromic acid, HBr, has a pH of 1.0. Hydrofluoric acid, HF, has a pH of 2.1. (a) Identify the weak acid. (b) Give the equation for its dissociation in water, with state symbols. (c) Explain why its pH is higher.

(a) The weak acid

(b) Its equation

Nudge (only if you are stuck): At the same concentration, the acid with fewer H⁺ ions has the higher pH.

6. identify

Supplement. Lemon juice gets its sour taste from citric acid, a weak acid. Which statement about the citric acid in lemon juice is correct?

Circle one answer.

It is not really an acid, because acids are dangerous to drink.
It has some H⁺ ions, but most of its molecules stay whole.
All its molecules have split into ions.
It has no H⁺ ions, because it is a weak acid.

Nudge (only if you are stuck): Weak means partially dissociated.

7. deduce / explain

Supplement. Two unlabelled bottles, U and V, hold hydrochloric acid and ethanoic acid, one in each, at the same concentration, volume and temperature. Identical pieces of magnesium ribbon are dropped into each. In U it fizzes fast; in V it fizzes slowly. (a) Deduce which bottle holds the weak acid, ethanoic acid. (b) Explain the difference, in terms of dissociation and hydrogen ions.

(a) The weak acid

(b) Why it fizzes more slowly

Nudge (only if you are stuck): Magnesium reacts with the H⁺ ions in the acid.