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, indicating that the reaction between citric acid and sodium hydrogencarbonate is endothermic. Initially, as the first amount of citric acid is added, energy is absorbed from the solution, causing a drop in temperature. After reaching a certain point, as more citric acid is added, the temperature stabilizes and may eventually rise again once the reaction completes.

The line should be less steep starting at 16.8 °C and reaching 1.00 g of citric acid since the metal container is a better conductor than the polystyrene cup. Therefore, more energy is absorbed from the surroundings, resulting in a smaller change in temperature.

The student would add citric acid to the flask slowly until there is a permanent colour change from the indicator. They should swirl the flask continuously during this process. The final and initial burette readings should then be measured to determine the volume of citric acid used.

1. A burette allows for precise measurement of the volume of citric acid added, enabling accurate determination of the end-point.
2. The burette allows the citric acid to be added drop by drop, which is essential for reaching the endpoint slowly and carefully.

To find the moles of citric acid used, we calculate:

ext{moles citric acid} = rac{13.3 imes 0.0500}{1000} = 0.000665

Then, since the reaction ratio is 3 NaOH : 1 C6H8O7, the moles of NaOH:

$ext{moles NaOH} = 3 imes 0.000665 = 0.001995$

Thus, the concentration of sodium hydroxide is:

ext{conc} = rac{1000}{25.0} imes 0.001995 = 0.0798 ext{ mol/dm}³