Acids & Bases — Practice · Grade 11 Science

Chapter: Acids, Bases & Titration

What this practice covers

By the end of Acids & Bases you should be able to:

9 questions, multiple choice and written answer, each with a worked explanation.

Vocabulary used: Arrhenius acid, Arrhenius base, Brønsted-Lowry acid, Brønsted-Lowry base, strong acid, weak acid.

Problem 1

An Arrhenius acid produces which ion when dissolved in water?
  1. hydrogen ion, H⁺ (as H₃O⁺)
  2. sodium ion, Na⁺
  3. chloride ion, Cl⁻
  4. hydroxide, OH⁻
Answer and explanation

hydrogen ion, H⁺ (as H₃O⁺)

Arrhenius acids release H⁺ ions, which combine with water to form hydronium, H₃O⁺.

Problem 2

In the Brønsted-Lowry model, a base is defined as a…
  1. proton donor
  2. proton acceptor
  3. electron donor
  4. hydroxide crystal
Answer and explanation

proton acceptor

A Brønsted-Lowry base accepts a proton (H⁺).

Problem 3

Which statement correctly describes a strong acid?
  1. It is safe to taste because it is common
  2. It cannot conduct electricity
  3. It ionizes completely in water, releasing nearly all its H⁺
  4. It always has a pH above 7
Answer and explanation

It ionizes completely in water, releasing nearly all its H⁺

A strong acid ionizes essentially completely, releasing almost all of its hydrogen ions into solution.

Problem 4

Explain the difference between a strong acid and a concentrated acid.
Answer and explanation

'Strong' describes how completely the acid ionizes in water — a strong acid ionizes almost 100%. 'Concentrated' describes how much acid is dissolved per volume (a high molarity). The two are independent: you can have a dilute solution of a strong acid or a concentrated solution of a weak acid.

Problem 5

Water reacts as a base in HCl + H₂O → H₃O⁺ + Cl⁻ but as an acid in NH₃ + H₂O → NH₄⁺ + OH⁻. Use the Brønsted-Lowry definitions to explain water's role in each reaction.
Answer and explanation

In the first reaction water accepts a proton from HCl (becoming H₃O⁺), so it acts as a Brønsted-Lowry base. In the second reaction water donates a proton to ammonia (becoming OH⁻), so it acts as a Brønsted-Lowry acid. A substance that can do both is called amphoteric.

Problem 6

Identify the conjugate base of H₂CO₃ after loss of one proton. Show the reasoning, not just the conclusion.
Hints

Acid/base roles depend on the stated reaction.

Answer and explanation

HCO₃⁻.

Problem 7

Can water act as both acid and base in different reactions?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Yes. It can donate or accept a proton depending on its reaction partner.

Problem 8

Strength and concentration answer different questionsStrength and concentration answer different questionsSelected solution at 25°CFormalconcentrationApproximate pHStrong monoprotic acid0.0010 M3.00Weak acid, Ka=1.0×10⁻⁵0.10 M3.00Strength concerns equilibrium ionization, not simply the pH value.The more concentrated weak acid can match a dilute strong acid.
Original rounded comparison with Ka=1.0×10⁻⁵ for a hypothetical weak acid. It is not an instruction to identify unknown chemicals by contact or taste.
Interpret the supplied visual. Can pH near 3 alone distinguish the two selected acid solutions?
Answer and explanation

No. Their formal concentrations and ionization equilibria differ but give similar hydronium concentration under the model.

Acid strength describes the extent of proton-transfer equilibrium under defined conditions, commonly summarized by Ka. Concentration specifies how much acid is present per solution quantity. A dilute strong acid can have a similar pH to a more concentrated weak acid because they produce similar hydronium activity. The selected comparison uses a monoprotic ideal model at 25°C. Strong does not mean concentrated, corrosive in every context or guaranteed to have the lowest pH among unspecified samples.

No. Their formal concentrations and ionization equilibria differ but give similar hydronium concentration under the model.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 9

In NH₃+H₂O⇌NH₄⁺+OH⁻, identify the acid, base and both conjugate pairs. Explain why NH₃ can act as a base without OH in its formula.
Answer and explanation

H₂O donates a proton and NH₃ accepts it. Pairs are H₂O/OH⁻ and NH₄⁺/NH₃. NH₃ produces hydroxide through proton transfer with water rather than having to release pre-existing OH⁻ groups.

H₂O donates a proton and NH₃ accepts it. Pairs are H₂O/OH⁻ and NH₄⁺/NH₃. NH₃ produces hydroxide through proton transfer with water rather than having to release pre-existing OH⁻ groups.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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