Consider the following statements : 1. Potassium carbonate is a salt of a weak acid and a weak base. 2. Ammonium nitrate is a salt of a strong acid and a weak base. 3. Ammonium acetate is a salt of a weak acid and a weak base. Which of the statements given above are correct?

Correct — B, 2 and 3 only. Name each salt by the acid and base it comes from. Ammonium nitrate comes from nitric acid, a strong acid, and ammonium hydroxide, a weak base — so statement 2 is correct, and such a salt gives an acidic solution. Ammonium acetate comes from acetic acid, a weak acid, and ammonium hydroxide, a weak base — so statement 3 is correct, and its solution is nearly neutral because the two hydrolyses cancel. Statement 1 is wrong: potassium carbonate comes from carbonic acid, which is weak, but from potassium hydroxide, which is a strong base, so it is a weak-acid strong-base salt and its solution is basic.

• (a)1 and 3 only — Keeps the true statement 3 but accepts the false one about potassium carbonate. Potassium hydroxide is one of the strongest common bases, so no salt of it can be called the salt of a weak base.
• (c)1 and 2 only — Correct on ammonium nitrate but wrong on potassium carbonate, and it also drops ammonium acetate, which is the textbook example of a weak-acid weak-base salt.
• (d)1, 2 and 3 — Would require potassium hydroxide to be a weak base. It is not, and potassium carbonate solution is distinctly alkaline, which is why washing soda's potassium analogue is used as a cleaning alkali.

Every salt can be traced back to the acid and base that would form it, and that ancestry decides the pH of its solution. A strong acid with a strong base gives a neutral salt such as sodium chloride. A strong acid with a weak base gives an acidic salt such as ammonium chloride or ammonium nitrate, because the cation hydrolyses. A weak acid with a strong base gives a basic salt such as sodium carbonate or sodium acetate, because the anion hydrolyses. A weak acid with a weak base gives a salt whose pH depends on the relative strengths, and ammonium acetate, where the two are almost equal, comes out nearly neutral.

The reliable method is to split each salt into its two ions and name the parent acid and base for each, rather than to judge the salt as a whole. Potassium is the tell here: all the alkali-metal hydroxides — lithium, sodium, potassium — are strong bases, so any potassium salt is a strong-base salt whatever its anion. Once statement 1 falls, only one option remains that keeps both of the other statements. This card marks a different letter from the answer we hold on file; the working above is the reason.

• Potassium carbonate is the salt of a weak acid (carbonic) and a strong base (potassium hydroxide), so its solution is basic.
• Ammonium nitrate is the salt of a strong acid (nitric) and a weak base (ammonium hydroxide), so its solution is acidic.
• Ammonium acetate is the salt of a weak acid and a weak base and is close to neutral in water.
• Hydroxides of the alkali metals — lithium, sodium, potassium — are all strong bases.
• Salt hydrolysis is the reason a neutral-looking salt solution can be acidic or alkaline.

• Judging a salt as a whole rather than splitting it into cation and anion.
• Assuming a carbonate must come from a weak base because carbonic acid is weak.
• Reading 'ammonium' as strong because ammonia is a familiar reagent; ammonium hydroxide is a weak base.

A three-statement classification item that is decided entirely by knowing which of the common acids and bases are strong.

• practice — not a real PYQ

  An aqueous solution of sodium carbonate is

  • (a)acidic
  • (b)neutral
  • (c)basic
  • (d)amphoteric

  Answer(c) basic — it is the salt of a weak acid and a strong base, so the carbonate ion hydrolyses and raises the pH.

• practice — not a real PYQ

  Which one of the following salts gives an acidic aqueous solution?

  • (a)Sodium chloride
  • (b)Ammonium chloride
  • (c)Sodium acetate
  • (d)Potassium nitrate

  Answer(b) Ammonium chloride — the salt of a strong acid and a weak base, so the ammonium ion hydrolyses to release hydrogen ions.