SRJC H2 Chem 2013 Prelim P2 Soln

1    1    2013 SRJC Prelim Paper H2 Chemistry P2 Suggested Solutions 1 Hard water, or water that contains high mineral content, such as calcium ions, pose serious problems in many industries. Other potential ions that could be present include zinc (through corrosion of pipings) and aluminium ions (whereby aluminium salts are added during the water purification process to start certain precipitation reactions). You are provided with a water sample that may contain the cations Ca 2+, Al3+ and Zn2+. The solution may also contain carbonate ions, CO 3 2–, and halide ions (either bromide or iodide). You are given only the following to conduct your test. Reagent Apparatus  aqueous ammonia  tests tubes  aqueous sodium hydroxide  funnel  dilute sulfuric acid  filter paper (a) Using the above only, write out a plan to separate each cation into three separate solutions. In your plan you should:  give a full description of the procedures you would use;  indicate the expected observations in each step. [5] Steps Test Observations 1 Add 1 cm 3 of the sample solution into a test tube. - 2 Add excess sodium hydroxide White ppt of Ca(OH) 2 will be produced. There may be slight dissolving of white ppt when NaOH is added in excess due to presence of A l3+ and Zn2+ ions. 3 Filter any precipitate being formed. White ppt of Ca(OH)2 will be collected as residue. Al3+ and Zn 2+ ions will be soluble and collected in the filtrate as Al(OH)4 – and Zn(OH)4 2- ions as a colourless solution. 4 Add dilute sulfuric acid Effervescence may be seen if CO3 2– is present. 5 Add aqueous ammonia in excess. White ppt of Al(OH)3 may be formed. White ppt of Zn(OH)2 may be formed, which is soluble in excess. 6 Filter any precipitate being formed. White ppt of Al(OH)3 will be collected as residue. Zn2+ ions will be soluble and collected in the filtrate as Zn(OH) 4 2- ions as a colourless solution. 1267

2    2    (b) A student proposed to use concentrated sulfuric acid instead of dilute sulfuric acid in (a). (i) Discuss how this change may affect the expected observations in (a). In the presence of a halide ion, concentrated sulfuric acid will act as an oxidising agent and react with the halide ions. This will interfere with the results as the coloured halogens produced may mask the white ppt of the metal hydroxides formed. (ii) Identify one potential safety hazard when the student make this change and state how you would minimise this risk. [4] Concentrated sulfuric acid is a corrosive acid. The student may adopt any one of the following : 1. Conduct the experiment in a fume hood 2. Wear safety goggles and gloves to prevent any contact with the eyes and skin. (c) You are also required to identify the anions present in the solution. (i) Propose two reagents to identify and distinguish between CO 3 2– and halide ion. For CO3 2–: dil