Silver nitrate solution reacts with sodium chloride solution to produce a white precipitate of silver chloride. The equation for the reaction is: AgNO₃ + NaCl → AgCl + NaNO₃. A student adds excess silver nitrate solution to a sample of sodium chloride solution. The student collects 2.87 g of dry silver chloride precipitate. The relative formula mass (Mr) of silver chloride is 143.5. The relative formula mass (Mr) of sodium chloride is 58.5. Explain how the student can calculate the mass of sodium chloride that reacted, and determine what that mass is.

Edexcel A-Level Chemistry (9CH0) — 5.1 Calculations using moles · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Silver nitrate solution reacts with sodium chloride solution to produce a white precipitate of silver chloride. The equation for the reaction is: AgNO₃ + NaCl → AgCl + NaNO₃. A student adds excess silver nitrate solution to a sample of sodium chloride solution and collects 2.87 g of dry silver chloride precipitate. The relative formula mass (Mr) of silver chloride is 143.5 and the relative formula mass (Mr) of sodium chloride is 58.5.

Model answer (5 marks)

1. Mass of AgCl = 2.87 g
2. Moles of AgCl = 2.87 g ÷ 143.5 g mol⁻¹ = 0.0200 mol
3. From the balanced equation AgNO₃ + NaCl → AgCl + NaNO₃, the molar ratio NaCl:AgCl = 1:1
4. Therefore moles of NaCl = 0.0200 mol
5. Mass of NaCl = 0.0200 mol × 58.5 g mol⁻¹ = 1.17 g

The student can calculate the mass of sodium chloride that reacted by converting the mass of the precipitate to moles, using the stoichiometric ratio, and then converting back to mass.

Examiner tips

  • Show the calculation steps and units clearly; use the 1:1 ratio from the balanced equation
  • State the moles of AgCl before converting to NaCl; keep significant figures consistent

Common mistakes

  • Using the wrong molar mass for AgCl or NaCl
  • Ignoring the 1:1 stoichiometric ratio and using the wrong conversion factor
  • Not showing the unit conversions or leaving out the final mass calculation

Mark scheme (5 marks)

  1. Divide the mass of silver chloride by its Mr to find moles of silver chloride
  2. Moles of silver chloride = 0.02 mol
  3. Use the 1:1 molar ratio from the equation to state that moles of NaCl = moles of AgCl
  4. Multiply moles of NaCl by its Mr to find mass of NaCl
  5. Mass of sodium chloride = 1.17 g

Key terms in this question

relative formula mass

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