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Blue crystals are dissolved in water. Sodium hydroxide gives a pale blue precipitate…
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Blue crystals are dissolved in water. Sodium hydroxide gives a pale blue precipitate. Excess ammonium hydroxide dissolves the precipitate to a deep blue solution. Barium chloride gives a white precipitate that does not dissolve in dilute hydrochloric acid. Hydrogen sulphide in dilute acid gives a black precipitate. The crystals come from this list of eight: copper(II) sulphate pentahydrate, copper(II) chloride dihydrate, nickel(II) sulphate heptahydrate, cobalt(II) chloride hexahydrate, iron(II) sulphate heptahydrate, zinc sulphate heptahydrate, lead(II) nitrate and manganese(II) sulphate tetrahydrate. Use Cu = 63.5, S = 32, O = 16 and H = 1 in g/mol. Find the number of salts that fit, the molar mass of the one that fits, its percentage of copper, and the mass of barium sulphate that 2.495 g of the crystals gives.
Need a reminder? The key idea
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The cation. A pale blue hydroxide that dissolves to a deep blue ammine, and a black sulphide in acid, point to copper(II).
Step 2. The anion. A white precipitate that stays in dilute acid with barium chloride is sulphate. So only copper(II) sulphate pentahydrate fits all five observations: 1 salt.
Step 3. Molar mass. .
Step 4. Copper. .
Step 5. Barium sulphate. of crystals, which gives of . The mass is .
Answer: 1 salt, , and .
Check. The copper chloride fails the barium chloride test, and the other salts fail the colour or the group II sulphide test.
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