WJEC Chemistry for AS: Student Bk

Gravimetric analysis Aim To measure the exact concentration of calcium ions in a calcium chloride solution by precipitation as calcium carbonate and weighing. Procedure Place exactly 100 cm 3 of an approximately 0.2 mol dm −3 calcium chloride solution into a clean beaker and stir in around 110 cm 3 of an approximately 0.2 mol dm −3 solution of sodium carbonate. Allow the precipitate to settle and check for complete precipitation by adding more drops of carbonate to the clear solution. When no further precipitate forms, filter off the precipitate making sure that all of it is transferred to the filter.* Wash the precipitate with distilled water to remove any impurities and dry in a drying oven. Weigh the dry precipitate plus filter and subtract the weight of the filter to give the mass of the calcium carbonate. Use this to calculate the mass of calcium in the 100 cm 3 sample and thus the exact concentration of the calcium chloride solution. *NB The filtration procedure will depend on the apparatus available. Most simply vacuum filtration though a sintered glass crucible is used but otherwise filter paper in a filter funnel is employed. Calculation of concentration Volume of Ca solution = V dm 3 Mass of precipitate/g = m (= mass of ppt + filter – mass filter) Mass of CaCO 3 /dm 3 = m/V Concn of calcium ions = m/VM where M is molar mass of CaCO 3 . The numbers of calcium ions in the carbonate and chloride solution are of course identical. PRACTICAL CHECK Gravimetric analysis and Qualitative analysis are specified practical tasks Qualitative analysis See questions 1,6,7 and 9 in Exam Practice Questions ▲ Forming CuSO 4 ▲ CuSO 4 crystals ▲ Adding CuO ▲ Evaporating CuSO 4 1.6 The periodic table 61