Preparation of salts

O Level / IGCSE Chemistry · Acids, bases and salts. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

The method used to make a salt depends on whether the salt dissolves in water, so the solubility rules come first.

A soluble salt is made from an acid. If the other reactant is insoluble (a metal, a base or a carbonate), add it in excess so that all the acid is used up, then filter off what is left over. If the other reactant is a soluble alkali, use titration to find the exact volumes that react. The salt is then crystallised from its solution.

An insoluble salt is made by precipitation: mix two solutions that each supply one of the ions needed, then filter, wash and dry the solid.

Rules to remember

Common mistake

Students choose the excess-solid method for sodium, potassium or ammonium salts. The alkali is soluble, so an excess cannot be filtered off; these salts need titration.

Worked example

14.3 g of hydrated sodium carbonate, Na2CO3·xH2O, is heated until all the water has gone. 5.3 g of anhydrous Na2CO3 is left. What is x? (H 1, C 12, O 16, Na 23)

  1. Mass of water lost = 14.3 − 5.3 = 9.0 g; moles of water = 9.0 ÷ 18 = 0.50 mol.
  2. Mr of Na2CO3 = (2 × 23) + 12 + (3 × 16) = 106; moles = 5.3 ÷ 106 = 0.050 mol.
  3. x = moles of water ÷ moles of salt = 0.50 ÷ 0.050 = 10.

Answer: x = 10, so the formula is Na2CO3·10H2O.

Practice questions

Try each one, then open the answer.

1. Which salt is insoluble in water?

  1. A
    barium sulfate
  2. B
    potassium nitrate
  3. C
    sodium chloride
  4. D
    copper(II) sulfate
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Answer: A. All nitrates and all sodium and potassium salts are soluble. Barium sulfate is insoluble, which is why it forms as a white precipitate in the sulfate test.

2. Which pair of solutions would be mixed to prepare the insoluble salt lead(II) iodide by precipitation?

  1. A
    lead(II) nitrate and sodium sulfate
  2. B
    lead(II) nitrate and potassium iodide
  3. C
    lead metal and iodine
  4. D
    lead(II) sulfate and sodium iodide
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Answer: B. Two soluble salts, one with the metal ion and one with the required anion: Pb(NO3)2 + 2KI → PbI2(s) + 2KNO3. The yellow precipitate is filtered, washed and dried.

3. Which method is suitable for preparing pure crystals of copper(II) sulfate?

  1. A
    mix copper(II) nitrate and sodium sulfate solutions, filter, then crystallise
  2. B
    warm copper metal with dilute sulfuric acid, filter off the excess, then crystallise
  3. C
    warm dilute sulfuric acid with excess copper(II) oxide, filter, then crystallise
  4. D
    titrate copper(II) hydroxide against sulfuric acid with an indicator, then crystallise
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Answer: C. Excess insoluble base is added to the acid so all acid reacts; the unreacted oxide is filtered off, and the solution is evaporated to crystallise the salt.

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