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

The initial decrease in temperature indicates that energy is being absorbed by the reaction, which is endothermic as citric acid reacts with sodium hydrogencarbonate. When 1.5 g of citric acid is added, the sodium hydrogencarbonate reacts completely, and the temperature stabilizes. As more citric acid is added, the temperature rises slightly, indicating that either the reaction is becoming less endothermic or additional heat from the surroundings is being absorbed.

The line should start at the same initial temperature as the first student's results at 16.8 °C and rise less steeply than the original line, reflecting the better thermal conduction of the metal container and reaching the same temperature at 1.00 g of citric acid. This sketch illustrates that the metal container absorbs heat more efficiently.

The student would add citric acid from the burette to the conical flask gradually, swirling the flask continuously. They would observe the color change in the indicator until a permanent color change is achieved, indicating the endpoint of the titration. Finally, the volume of citric acid used would be recorded.

1. A burette allows for precise measurement and control of the volume of citric acid added drop by drop.
2. It enables the user to easily read the volume of liquid remaining and measure the total volume dispensed accurately.

To calculate the concentration of sodium hydroxide, first calculate moles of citric acid used:

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

Using the reaction ratio from the equation, moles of NaOH needed:

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

Now, calculating concentration of NaOH:

$ext{concentration} = \frac{0.001995}{0.025} = 0.0798 ext{ mol/dm³}$