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Preparing a transition metal complex (Core Practical 14)
Edexcel IAL Chemistry · WCH16 Practical Skills in Chemistry II
Core Practical 14 prepares a transition metal complex as crystals and works out the yield. The ligand exchange chemistry is in the Unit 5 lesson; here you practise explaining each step.
A typical preparation
Tetraamminecopper(II) sulfate, [Cu(NH3)4]SO4·H2O:
1. Dissolve a weighed mass of copper(II) sulfate-5-water in the minimum volume of water.
2. In a fume cupboard, add concentrated ammonia until the pale blue precipitate first formed dissolves to a deep blue solution.
3. Add ethanol: the complex is much less soluble in ethanol than in water, so it crystallises.
4. Cool in ice, then filter under reduced pressure.
5. Wash the crystals with a little cold ethanol (not water, which would dissolve them; ethanol also evaporates quickly) and dry.
Pearson's guide also gives iron(II) ethanedioate-2-water, FeC2O4·2H2O, as an option.
Yield and its losses
- Theoretical mass from the reacting ratio (here 1 : 1 copper), then % yield = actual / theoretical x 100.
- Losses: some complex stays dissolved, crystals left on apparatus, incomplete precipitation.
- A yield above 100% usually means the crystals are still wet; dry them (overnight if needed) before weighing.
Hazards
- Concentrated ammonia: corrosive, with an irritating vapour; use a fume cupboard and eye protection.
- Ethanol: flammable; no naked flames.
- Copper(II) compounds: harmful and hazardous to aquatic life; dispose of them properly.
Worked example
2.50 g of CuSO4·5H2O (M = 249.6) gives 1.85 g of [Cu(NH3)4]SO4·H2O (M = 245.6). Calculate the % yield.
- n(Cu) = 2.50 / 249.6 = 0.01002 mol
- Theoretical mass = 0.01002 x 245.6 = 2.46 g
- % yield = 1.85 / 2.46 x 100
Answer: 75.2%
Check yourself
- Calculate the maximum mass of [Cu(NH3)4]SO4·H2O (M = 245.6) from 2.50 g of CuSO4·5H2O (M = 249.6).
Show answer
2.46 g. 0.01002 mol x 245.6 = 2.46 g.
- The actual mass obtained is 1.85 g. Calculate the percentage yield (theoretical mass 2.46 g).
Show answer
75.2 %. 1.85 / 2.46 x 100 = 75.2%.
- Why is ethanol added to the deep blue solution?
- Ethanol is a ligand
- The complex is less soluble in ethanol, so it crystallises out
- Ethanol oxidises the copper
- Ethanol dries the solution
Show answer
The complex is less soluble in ethanol, so it crystallises out. Lower solubility makes the solid come out of solution.
- Why are the crystals washed with cold ethanol rather than water?
- Water would react with ammonia
- Ethanol is cheaper
- The complex would dissolve in water; ethanol removes impurities and evaporates quickly
- Water is flammable
Show answer
The complex would dissolve in water; ethanol removes impurities and evaporates quickly. Washing must not dissolve the product.
- Why is concentrated ammonia added in a fume cupboard?
- Its vapour is corrosive and irritating
- It is flammable
- It explodes with copper sulfate
- It is an oxidising agent
Show answer
Its vapour is corrosive and irritating. Ammonia vapour irritates the eyes and lungs.
- A student reports a yield of 108%. What is the most likely reason?
- Some product was lost during filtration
- Too little ammonia was used
- The crystals were not fully dry when weighed
- The reaction was incomplete
Show answer
The crystals were not fully dry when weighed. Water adds mass.
- What is the advantage of filtering under reduced pressure?
- It is faster and leaves the crystals drier
- It dissolves impurities
- It makes larger crystals
- It increases the theoretical yield
Show answer
It is faster and leaves the crystals drier. Suction pulls the liquid through quickly.
Sources
Last updated 8 November 2026. Spotted a mistake? Tell us.