After a precipitate forms, use the ionic equation to work out how much of each reacting ion is used. The ions that take no part in the precipitation remain dissolved. If a reacting ion starts in excess, some of that ion can remain dissolved too.
The 2027 SEC G3 Chemistry K324 syllabus includes ionic equations and stoichiometry. This example uses silver chloride formation to connect the equation with the ions expected in the filtrate after the solid is removed.
- Identify the ions in the starting solutions.
- Write the net ionic equation.
- Use the reacting ratio to find any excess.
- Include spectator ions when describing the filtrate.
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Identify the reacting pair
Mixing silver nitrate solution with sodium chloride solution provides Ag⁺, NO₃⁻, Na⁺ and Cl⁻ ions. The net ionic equation is Ag⁺(aq) + Cl⁻(aq) → AgCl(s). Silver ions and chloride ions are removed from solution in equal amounts as the solid forms.
Sodium and nitrate ions appear on both sides of a complete ionic equation. They remain dissolved during this reaction. The net ionic equation omits them because they do not undergo a chemical change.
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Worked example | Chloride is in excess
Suppose the silver nitrate solution provides 0.0030 mol of Ag⁺ and 0.0030 mol of NO₃⁻. The sodium chloride solution provides 0.0050 mol of Na⁺ and 0.0050 mol of Cl⁻. Treat the precipitation as complete for the calculation. The silver ions use 0.0030 mol of chloride ions and form 0.0030 mol of AgCl.
After filtration, the liquid contains 0.0050 mol of Na⁺, 0.0030 mol of NO₃⁻ and 0.0020 mol of excess Cl⁻ in this model. The solid AgCl stays on the filter paper. The dissolved chloride listed here is the chloride left over after precipitation.
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CHAPTER 3 OF 4 · Back to contents
Use charge to check the result
The positive charge from 0.0050 mol of Na⁺ matches the total negative charge from 0.0030 mol of NO₃⁻ and 0.0020 mol of Cl⁻. This check can reveal an omitted ion. Try swapping the starting amounts of the two salts. The excess reacting ion would then be Ag⁺, with 0.0020 mol remaining in the same idealised model.
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CHAPTER 4 OF 4 · Back to contents
Frequently asked questions
Is excess chloride a spectator ion?
Chloride takes part in precipitation. The amount that exceeds the silver available remains dissolved. Sodium and nitrate are the spectator ions.
Does filtering remove dissolved ions?
Ordinary filtration separates the solid precipitate from the liquid. The dissolved ions pass through with the filtrate.