12 (B410U10-1) Examiner only 13. (a) A student was told that a bottle of hydrochloric acid he was given was approximately of concentration 0.1 mol dm−3. He was asked to determine the accurate concentration of the acid using a titration method. He had access to solid anhydrous sodium carbonate, Na2CO3, and the apparatus normally found in a school or college laboratory. (i) Write the equation for the reaction between sodium carbonate and hydrochloric acid. [1] (ii) Describe how the student should carry out the determination of the concentration of the acid. You should include the apparatus required and the masses and volumes of any chemicals used. I. Outline the preparation of a solution of sodium carbonate suitable for use in a titration against the hydrochloric acid. [3] II. Describe how the student should carry out the titration to determine the concentration of the acid. You do not need to describe how the apparatus is cleaned or set up. [3] © WJEC CBAC Ltd. (B410U10-1) Turn over. 13 Examiner only (b) The student found in (a) that the hydrochloric acid had a concentration of 0.110 mol dm−3. Calculate the pH of this solution. [2] pH = ............................................... (c) Another student was given a sample of ore containing compounds of silver and was asked to find the percentage of silver in the ore. He dissolved 2.48 g of the silver ore in nitric acid and added excess of the hydrochloric acid to precipitate the silver as silver chloride. He filtered off the precipitate of silver chloride and found its mass. (i) State how the precipitate was treated in order to obtain the mass of the silver chloride. Explain why the precipitate was treated in this way. [2] (ii) Write the ionic equation for the formation of this precipitate. Include state symbols. [1] (iii) The mass of silver chloride obtained was 0.93 g. Calculate the percentage of silver in the ore. [2] Percentage = ............................................... % © WJEC CBAC Ltd. 14