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This question is about citric acid (C6H8O7) - AQA - GCSE Chemistry - Question 9 - 2020 - Paper 1
Question 9
This question is about citric acid (C6H8O7). Citric acid is a solid. A student investigated the temperature change during the reaction between citric acid and sodi... show full transcript
Worked Solution & Example Answer:This question is about citric acid (C6H8O7) - AQA - GCSE Chemistry - Question 9 - 2020 - Paper 1
Step 1
Step 2
Explain the shape of the graph in terms of the energy transfers taking place.
Answer
The graph shows a decrease in temperature as citric acid is added initially because the reaction between citric acid and sodium hydrogen carbonate is endothermic. The temperature continues to drop until all the sodium hydrogen carbonate has reacted. As more citric acid is added, the temperature begins to stabilize because the reaction has completed, resulting in less energy being taken from the solution, which can be seen when the temperature reaches around 11.6°C.
Step 3
Sketch a line on Figure 6 to show the second student's results until 1.00 g of citric acid had been added. Explain your answer.
Answer
The line for the second student's results would be less steep starting at 16.8°C and reaching the same conclusion at around 1.00 g of citric acid added. This is because metal is a better conductor than a polystyrene cup, meaning that more energy is absorbed from the surroundings, which causes less dramatic temperature drops.
Step 4
Calculate the mass of citric acid (C6H8O7) required.
Answer
To calculate the mass of citric acid required:
- Molar mass of citric acid (C6H8O7) = 6(12) + 8(1) + 7(16) = 192 g/mol.
- Moles of citric acid = concentration × volume = 0.0500 mol/dm³ × (250 cm³ ÷ 1000) = 0.0125 moles.
- Mass = moles × molar mass = 0.0125 × 192 = 2.4 g.
Step 5
Describe how the student would complete the titration.
Answer
The student would add the citric acid slowly to the flask while swirling until the indicator shows a permanent color change. They would record the final burette reading and may repeat this process to calculate a mean volume of citric acid used.
Step 6
Step 7
Calculate the concentration of the sodium hydroxide in mol/dm³.
Answer
Using the equation:
Moles of citric acid = 13.3 × 0.0500 / 1000 = 0.000665 moles. Moles of NaOH required = 3 × 0.000665 = 0.001995 moles. Concentration of NaOH = moles/volume = 0.001995 moles / (25.0 cm³ / 1000) = 0.0798 mol/dm³.