This question is about citric acid (C6H8O7) - AQA - GCSE Chemistry - Question 9 - 2020 - Paper 1

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.

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.

To calculate the mass of citric acid required:

1. Molar mass of citric acid (C6H8O7) = 6(12) + 8(1) + 7(16) = 192 g/mol.
2. Moles of citric acid = concentration × volume = 0.0500 mol/dm³ × (250 cm³ ÷ 1000) = 0.0125 moles.
3. Mass = moles × molar mass = 0.0125 × 192 = 2.4 g.

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.

1. A burette allows for precise measurement of the solution volume.
2. A burette can add the solution drop by drop, which helps in observing the endpoint of the titration more accurately.

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.