Hydrochloric acid, HCl, is neutralised when it is added to aqueous potassium hydroxide, KOH. KOH + HCl → KCl + H₂O The reaction is exothermic. P is aqueous potassium hydroxide. Q is 2.24 mol/dm³ hydrochloric acid. Read all the instructions carefully before starting the experiments. Instructions You are going to do six experiments.

1. (a)Experiment 1 • Rinse and fill a burette with Q. • Place the plastic cup into a beaker. • Use a volumetric pipette to add 25.0cm³ of P to the plastic cup. • Use a measuring cylinder to add 20 cm³ of water to the plastic cup. • Stir the mixture in the cup with the thermometer and measure its temperature to the nearest 0.5°C. • Record this initial temperature in column E of Table 1.1. • Use the burette to add 5.0cm³ of Q to the plastic cup whilst stirring. • Measure the highest temperature reached. • Record this value in column F of Table 1.1. • Empty the plastic cup and rinse it with water. Experiments 2–6 • Repeat Experiment 1 using the volumes of water and Q shown in columns C and D of the table. Refill the burette as necessary. • Calculate the temperature rise for each of Experiments 1–6 and record them in column G of Table 1.1. Table 1.1 A B C D E F G experiment number volume of P /cm³ volume of water /cm³ volume of Q /cm³ initial temperature of mixture /°C highest temperature reached /°C temperature rise /°C 1 25.0 20 5.0 2 25.0 15 10.0 3 25.0 10 15.0 4 25.0 7 18.0 5 25.0 5 20.0 6 25.0 0 25.0[8]
2. (b)Draw a graph of temperature rise against volume of Q on the grid in Fig. 1.1. You should: • plot the point (0,0) as there is no temperature rise when no Q is added • plot temperature rise (column G) against volume of Q (column D) from Experiments 1–6 • draw a straight line of best fit for the first three points • draw a straight line of best fit for the last four points • extend the lines so that they intersect.[3]
3. (c)The point where the two lines intersect indicates the volume of Q that exactly neutralises 25.0 cm³ of P. Determine the volume of Q where the two lines on the graph intersect. volume of Q ..................................................... cm³[1]
4. (d)Q is 2.24 mol/dm³ hydrochloric acid. Use your answer to (c) to calculate the concentration of potassium hydroxide in P. KOH + HCl → KCl + H₂O concentration of potassium hydroxide in P .............................................. mol/dm³[2]
5. (e)State and explain why a plastic cup rather than a metal cup is used in these experiments.[2]
6. (f)A measuring cylinder is used to measure the volumes of water. Suggest why the volumes of water in Table 1.1 are not given to one decimal place.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.