Beatty 1. VA1 Paper 3 Prelims 2019_Answers

1 Beatty Secondary School Science Department (Chemistry Unit) Chemistry 6092 Name: ___________________________________ ( ) Date: ________________ Class: 4E___ Practical 1: VA1 – Acid-Base Titration (Beatty Prelim 2019) 1 Antacids contain calcium carbonate that can neutralise the acid in your stomach to relieve indigestion and heartburn. CaCO3 + 2HCl → CaCl2 + H2O + CO2 You are going to determine the mass of calcium carbonate in one antacid tablet. Excess acid is added to an antacid tablet. The volume of the unreacted acid is then determined by a titration with an alkali. In this experiment, one antacid tablet is reacted with 250 cm3 of 0.500 mol/dm3 hydrochloric acid, to form solution P. Solution P contains the excess hydrochloric acid that did not react with the antacid tablet. Solution Q is 0.40 mol/dm3 sodium hydroxide. (a) (i) Put P into the burette. Pipette 25.0 cm3 of solution Q into a conical flask. Add solution P from the burette until the end point is reached. The end point is indicated by the colour change of the indicator. Record your titration results in the space provided, repeating the titration as many times as you consider necessary to achieve consistent results. Results titration number 1 2 3 final burette reading / cm3 23.40 23.10 23.10 initial burette reading / cm3 0.00 0.00 0.00 volume of solution P added / cm3 23.40 23.10 23.10 best titration results ✓ ✓ [5] 21

2 (ii) From your titration results, obtain an average volume of P to be used in your calculations. Show clearly how you obtained this volume. Average volume of P used = (23.10 + 23.10) ÷ 2 = 23.10 cm3 (2 decimal places) average volume of P 23.10 cm3 [1] (b) Q is 0.40 mol/dm3 sodium hydroxide. Calculate the number of moles of sodium hydroxide present in 25.0 cm3 of solution Q. No of mol of Q = 0.40 × 25 1000 = 0.0100 mol (3 sf) number of moles of sodium hydroxide 0.0100 mol [1] (c) Using your answer from (b), calculate the number of moles of hydrochloric acid that reacted with 25.0 cm3 of Q. Compare mole ratio: NaOH : HCl 1 : 1 0.01 : 0.01 number of moles of acid that react with Q 0.0100 mol [1] (d) Using your answer from (c), calculate the number of moles of hydrochloric acid in 250 cm3 of P. By proportion: 23.10 cm3 ------------ 0.0100 mol 1 cm3 ------------ 0.01 22.90 250 cm3 ------------ 0.01 25022.90  = 0.10822 mol number of moles of acid in 250 cm3 of P 0.108 mol [1] 23.10 23.10

3 (e) Calculate the number of moles of hydrochloric acid in 250 cm3 of 0.500 mol/dm3 hydrochloric acid. No of mol of HCl = 0.5 × 250 1000 = 0.125 mol (3 sf) number of moles in 250 cm3 of hydrochloric acid 0.125 mol [1] (f) Using your answer from (d) and (e), calculate the number of moles of hydrochloric acid that reacted with calcium carbonate in one antacid tablet. (e) –