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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
Initially, as citric acid is added, it reacts with the sodium hydrogen carbonate, taking energy from the solution, which is observed as a decrease in temperature. The graph slopes downward because the reaction is endothermic, meaning energy is taken in from the surroundings during the reaction. As more citric acid is added and all sodium hydrogen carbonate reacts, the temperature starts to stabilize.
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. The starting temperature of the solution was the same.
Answer
The line would be less steep than the original, starting at the same initial temperature. This is because metal is a better conductor than polystyrene, allowing more energy to be absorbed from the surroundings. Therefore, the temperature would decrease less with the same amount of citric acid added compared to the original experiment.
Step 4
Calculate the mass of citric acid (C6H8O7) required.
Answer
To find the mass of citric acid required, we need to calculate the number of moles:
Number of moles = volume (L) × concentration (mol/dm³)
= 0.250 L × 0.0500 mol/dm³ = 0.0125 mol.
Molar mass of citric acid (C6H8O7) = (6×12) + (8×1) + (7×16) = 192 g/mol.
Mass = moles × molar mass = 0.0125 mol × 192 g/mol = 2.4 g.
Step 5
Describe how the student would complete the titration.
Answer
The student would add the citric acid solution to the flask until the endpoint is reached, where the indicator changes color permanently. They should swirl the flask gently during the process to mix the solutions thoroughly and ensure an accurate endpoint is determined.
Step 6
Give two reasons why a burette is used for the citric acid solution.
Answer
- A burette allows for precise measurement of the volume of citric acid being added, ensuring the solution is dispensed gradually.
- It enables the student to add the citric acid drop by drop, which is crucial for accurately identifying the endpoint of the titration.
Step 7
Calculate the concentration of the sodium hydroxide solution in mol/dm³.
Answer
From the equation, 3 moles of NaOH react with 1 mole of citric acid. We already calculated the moles of citric acid used in this neutralization (0.001995 mol).
Moles of NaOH = 3 × 0.001995 mol = 0.005985 mol.
Volume of NaOH = 25.0 cm³ = 0.0250 L.
Concentration of NaOH = moles/volume in L = 0.005985 mol / 0.0250 L = 0.2394 mol/dm³.