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2.7.7Inorganic Chemistry

Prepare an Insoluble Salt — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 21 of 23 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Part 1 of 6Precipitation Reactions

This method works only when the salt to be made is insoluble in water.

Idea
  • Idea Two soluble salt solutions are mixed, and the ions that form an insoluble compound join and appear as a solid precipitate.
General pattern
  • General pattern Soluble salt 1 + soluble salt 2 → insoluble salt + soluble salt 3, which is also called double decomposition.
Example
  • Example The balanced equation is Pb(NO₃)₂(aq) + K₂SO₄(aq) → PbSO₄(s) + 2KNO₃(aq).
Ionic equation
  • Ionic equation Pb²⁺(aq) + SO₄²⁻(aq) → PbSO₄(s) shows only the ions that react.

Part 2 of 6Choosing the Reactants

Choosing the Reactants
Nitrates
  • Nitrates All nitrates are soluble, so lead(II) nitrate or silver nitrate is a common source of the metal ion.
Sulfates
  • Sulfates Most sulfates are soluble, but lead(II), barium and calcium sulfates are insoluble.
Chlorides
  • Chlorides Most chlorides are soluble, except silver and lead(II) chloride.
Carbonates and hydroxides
  • Carbonates and hydroxides Most are insoluble, except those of sodium, potassium and ammonium.
To makeSolution 1Solution 2
Lead(II) sulfateLead(II) nitratePotassium sulfate
Silver chlorideSilver nitrateSodium chloride
Barium sulfateBarium chlorideSodium sulfate

Part 3 of 6Method

Filtering, washing and drying purify the product.

1
Measure a fixed volume of one salt solution into a beaker.
2
Add the second solution until it is in slight excess so the maximum precipitate forms.
3
Filter the mixture so the solid stays in the filter paper.
4
Wash the residue with distilled water to remove soluble impurities.
5
Dry the precipitate in a warm oven or between filter papers.

Part 4 of 6Why Each Step Matters

Why Each Step Matters
Slight excess
  • Slight excess Making sure one reactant is in excess ensures all of the other has reacted, maximising yield.
Washing
  • Washing Washing removes the soluble salt solution left on the precipitate, which would contaminate it.
Distilled water
  • Distilled water Distilled water is used because tap water contains dissolved ions that could add impurities.

Part 5 of 6Worked Example

Question

  • 25.0 cm³ of 0.50 mol/dm³ lead(II) nitrate is mixed with excess potassium sulfate.
  • Calculate the maximum mass of lead(II) sulfate formed.
  • (Ar: Pb = 207, S = 32, O = 16)

  1. Moles of Pb(NO₃)₂ = 0.50 × 25.0 ÷ 1000 = 0.0125 mol.
  2. From the equation, 1 mol Pb(NO₃)₂ gives 1 mol PbSO₄, so moles of PbSO₄ = 0.0125 mol.
  3. Mr of PbSO₄ = 207 + 32 + (4 × 16) = 303.
  4. Mass = 0.0125 × 303 = 3.79 g.

Answer Maximum mass of PbSO₄ = 3.79 g

Part 6 of 6Exam Checklist & Tips

2.7.7 – Key Points for the Exam

  • Precipitate An insoluble solid formed when two solutions react.
  • Filtrate The liquid that passes through the filter paper.
  • Residue The solid left on the filter paper.
  • Insoluble Does not dissolve in water.

Common Mistakes

  • Choosing an insoluble reactant, since both starting salts must be soluble.
  • Skipping the washing step and leaving soluble impurities on the product.
  • Trying to crystallise the filtrate to obtain the insoluble salt.
  • Writing the precipitate as (aq) instead of (s) in equations.

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