A 25.0 mL sample of old ether is shaken with excess acidified potassium iodide…
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A sample of old ether is shaken with excess acidified potassium iodide. The peroxide oxidises iodide to iodine: . The iodine is titrated with sodium thiosulphate, and is needed. Two thiosulphate ions react with each . Find the amount of thiosulphate in micromoles, the amount of peroxide in micromoles, and the peroxide in mg per litre of ether, expressed as hydrogen peroxide (molar mass ).
Need a reminder? The key ideaShow
One mole of an ideal gas occupies : at and . The lower flammable limit of ether vapour is by volume. A titration gives moles of thiosulphate, and two thiosulphate ions react with each . An activating group directs to the ortho and para carbons.
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.
Given. of ether, of thiosulphate.
Step 1. Thiosulphate. .
Step 2. Iodine and peroxide. . One ROOH gives one , so the peroxide is .
Step 3. As hydrogen peroxide. in . In a litre: .
Check. A reading of this size means peroxide is present in the ether. Many laboratory safety guides call a level of about or more unsafe to distil. This sample should be treated before any distillation.
Answer: , and .
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