2.4.7Inorganic Chemistry: Part 1
How to Prepare an Insoluble Salt
5 min read 6 parts Lesson 19 of 20 Unit quiz
Section 1 of 6Precipitation
Precipitate
- An insoluble solid formed when two solutions are mixed is called a precipitate.
Two soluble salts
- The method mixes a solution of one soluble salt with a solution of another to give the insoluble salt.
General pattern
- AB(aq) + CD(aq) → AD(s) + CB(aq), where AD is the insoluble salt.
Ionic equation
- Only the ions that join to form the solid are shown, for example Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
Double decomposition
- This kind of reaction, in which the ions swap partners, is also called double decomposition.
Section 2 of 6Choosing the Reactants
Choosing the Reactants
Nitrates
- All nitrates are soluble, so metal nitrates are often used as one starting solution.
Sodium, potassium and ammonium
- Salts of these ions are all soluble, so they supply the other ion needed.
Chlorides
- Most chlorides are soluble, but silver chloride and lead(II) chloride are insoluble.
Sulfates
- Most sulfates are soluble, but barium sulfate, lead(II) sulfate and calcium sulfate are insoluble.
Carbonates and hydroxides
- Most carbonates and hydroxides are insoluble, except those of sodium, potassium and ammonium.
Section 3 of 6Method
1
Mix measured volumes of the two soluble salt solutions in a beaker and stir; a precipitate forms.
2
Make sure one solution is added in slight excess so that all of the other ion is used up.
3
Filter the mixture, so that the precipitate stays on the filter paper.
4
Wash the precipitate on the filter paper with distilled water to remove soluble impurities.
5
Dry the solid in a warm oven or between sheets of filter paper.
| Insoluble salt | Possible reactants |
|---|---|
| Barium sulfate | Barium chloride + sodium sulfate |
| Silver chloride | Silver nitrate + sodium chloride |
| Lead(II) iodide | Lead(II) nitrate + potassium iodide |
Section 4 of 6Quantities and Safety
Theoretical mass
- Work out the moles of the limiting reactant and use the equation to find the mass of precipitate expected.
Percentage yield
- Compare the dry mass actually obtained with the theoretical mass.
Toxic compounds
- Lead and barium compounds are toxic, so gloves and eye protection are needed and waste must be disposed of properly.
Washing
- Distilled water is used so that no new ions are introduced to the precipitate.
Section 5 of 6Worked Example
Question
- 40.0 cm³ of 0.100 mol/dm³ barium chloride is mixed with excess sodium sulfate solution: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq).
- Calculate the theoretical mass of barium sulfate.
- (Ar: Ba = 137, S = 32, O = 16)
- Moles of BaCl₂ = 0.100 × 0.0400 = 0.00400 mol.
- BaCl₂ and BaSO₄ are in a 1 : 1 ratio, so moles of BaSO₄ = 0.00400 mol.
- Mr of BaSO₄ = 137 + 32 + (4 × 16) = 233.
- Mass = 0.00400 × 233 = 0.932 g.
Answer The theoretical mass of barium sulfate is 0.932 g.
Section 6 of 6Exam Checklist & Tips
2.4.7 – Key Points for the Exam
- Precipitate An insoluble solid formed in a reaction between solutions.
- Insoluble Unable to dissolve in a given solvent such as water.
- Filtration Separating an insoluble solid from a liquid using filter paper.
- Ionic equation An equation that shows only the ions that take part in a reaction.
- Double decomposition A reaction between two compounds in which the ions swap partners.
Common Mistakes
- Choosing a starting solution containing an insoluble compound, such as a carbonate of a metal other than sodium.
- Forgetting to wash the precipitate, so soluble salt remains on the solid.
- Trying to obtain the solid by crystallisation, when it should be filtered off.
- Including state symbols incorrectly, such as writing the precipitate as (aq).