A student plans an experiment to investigate the yield of propanoic acid when a sample of propan-1-ol is oxidised - AQA - A-Level Chemistry - Question 6 - 2021 - Paper 3
Question 6
A student plans an experiment to investigate the yield of propanoic acid when a sample of propan-1-ol is oxidised.
Figure 4 shows the apparatus that the student pla... show full transcript
Worked Solution & Example Answer:A student plans an experiment to investigate the yield of propanoic acid when a sample of propan-1-ol is oxidised - AQA - A-Level Chemistry - Question 6 - 2021 - Paper 3
Step 1
Give two reasons why the apparatus shown in Figure 4 is not safe.
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Answer
The flask is not clamped, posing a risk of it falling and breaking.
The system is not sealed, which can lead to the escape of vapors and no control over pressure.
Step 2
Give one additional reagent that is needed to form any propanoic acid.
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Sulfuric acid needs to be added as a catalyst for the oxidation of propan-1-ol to form propanoic acid.
Step 3
State two more mistakes in the way the apparatus is set up in Figure 4.
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The direction of water flow through the condenser is incorrect; it should enter from the bottom.
A thermometer is not needed in this setup as it does not measure the necessary temperature accurately.
Step 4
State the purpose of the small glass beads in the flask in Figure 4.
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The small glass beads are used to prevent 'bumping' by providing a surface for the liquid to boil smoothly.
Step 5
After correcting the mistakes, the student heats a reaction mixture containing 6.50 g of propan-1-ol with an excess of the oxidising agent. The propanoic acid separated from the reaction mixture has a mass of 3.25 g. State the name of the technique used to separate the propanoic acid from the reaction mixture. Calculate the percentage yield of propanoic acid.
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Answer
The technique used to separate propanoic acid is fractional distillation.
To calculate the percentage yield of propanoic acid, use the formula:
Theoretical yield for 6.50 g of propan-1-ol is based on its molar mass (60.1 g/mol) and stoichiometry:
Moles of propan-1-ol = \frac{6.50}{60.1} = 0.108 moles
Moles of propanoic acid produced = 0.108 moles
Mass of propanoic acid produced = 0.108 \times 74.1 = 8.00 g
Now, using the formula:
Percentage yield=(8.00g3.25g)×100=40.625%≈40.6%.
Step 6
State a simple chemical test that distinguishes the propanoic acid from the propan-1-ol. Give one observation for the test with each substance.
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Test: Add sodium carbonate.
Observation: For propanoic acid, effervescence of bubbles will occur due to the release of carbon dioxide. For propan-1-ol, no visible reaction will occur.
