The objectives of this laboratory are: a) To experimentally determine the mass percent of oxygen in the compound potassium chlorate (KClO3) via the thermal decomposition of a sample of potassium chlorate. b) To qualitatively demonstrate that the residue resulting from the decomposition of potassium chlorate is potassium chloride.
All compounds consist of elements chemically combined in fixed proportions – they obey the Law of Constant Composition. One way to express the proportion each of element in a compound is as a percentage by mass, or mass percent. In Part A of this lab, a sample of potassium chlorate will be experimentally analyzed in order to determine the mass percent of elemental oxygen present in it. To do this, the potassium chlorate must be heated to temperatures greater 400 °C, causing it to thermally decompose into potassium chloride and free oxygen: heat
2 KClO3 (s)
2 KCl (s)
3 O2 (g)
Students will perform a quantitative analysis of the reactants and products of this reaction, measuring the initial mass of solid potassium chlorate used (before heating), and the mass of the solid potassium chloride product, or residue, remaining after heating. Applying the Law of Mass Conservation, the difference in these measured masses is the mass of oxygen released (from the original potassium chlorate sample). From this data, the experimental mass percent of oxygen in potassium chlorate will be determined: Mass of Oxygen Released x 100 Mass of Potassium Chlorate Used
Mass Percent of Oxygen (experimental) =
Mass percentages of elements in compounds can also be theoretically calculated using molar masses, along with the known chemical formula of the compound. Thus, the theoretical mass percent of oxygen in potassium chlorate would be calculated using the expression: 3 (Molar Mass of O) Molar Mass of KClO3
Mass Percent of Oxygen (theoretical)
Students can therefore evaluate their accuracy in this experiment by comparing their experimental results to the true theoretical value, and by calculating their percent error. Page 1 of 4
In Part B of this lab, the residue left after heating will be qualitatively analyzed in order to demonstrate that it is chemically different from the initial potassium chlorate sample. Specifically, the residue will be tested for the presence of chloride ions by the addition of nitric acid and aqueous silver nitrate. A positive test is indicated by the formation of a white precipitate. The actual identity of the residue will then be conclusively verified by comparing this result to those obtained for identical tests on known samples of potassium chlorate and potassium chloride.
Safety Be especially careful when using the Bunsen burner and handling hot equipment. Remember that most items look exactly the same whether they are hot or cold. Heat the potassium chlorate sample slowly to avoid any splattering. Be aware that silver nitrate may stain the skin and nitric acid may burn the skin. If a spill of either chemical occurs, rinse under running water and report the accident to your instructor. Nitric acid spills may also be neutralized using the sodium bicarbonate solution by the sinks. Materials and Equipment Solid potassium chlorate (KClO3), solid potassium chloride (KCl), 6M nitric acid (HNO3), 0.1M silver nitrate (AgNO3), two crucibles with lids, stand and ring clamp, clay triangle, crucible tongs, Bunsen burner, three medium-sized test tubes, test tube rack, stirring rod, and an electronic balance. Part A: Mass Percent of Oxygen in Potassium Chlorate The following steps should be carried out for two separate samples of potassium chlorate. 1. Clean both crucibles and their lids (obtained from the stockroom) by thoroughly rinsing with distilled water then drying as completely as possible with a paper towel. 2. Weigh the...