Determination of a Chemical Formula

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Akruti Patel
Lab Report #4: Determination of a chemical formula: the empirical formula of Magnesium Oxide 1. Purpose: Determine the empirical formula of magnesium oxide from the percent composition (this can found using the Analytical Method and the Synthesis Method). 2. Introduction: In the late eighteenth century, combustion has been studied extensively. In fact, according to Steven and Susan Zumdahl, Antoine Lavoisier, a French Chemist, performed thousands of combustion experiments and measured masses of every single reactant and product, including those which were gases (for example, Carbon Dioxide, Nitrogen, Hydrogen, and Oxygen). Lavoisier considered measurements to be an essential tool for chemistry. He observed that as the physical and chemical properties of the products and the reactants differed, the total mass of the products was always the same as the total mass of the reactants. His experiments suggested that “in a chemical reaction, mass is neither created nor destroyed” as summarized in the law of conservation of mass. (Zumdahl and Zumdahl 41) This experiment demonstrates the law of conservation of mass by and how it can be used to determine the empirical formula of magnesium oxide (MgO). The empirical formula is the simplest number ratio of each element in a substance. In order to get the empirical formula, the magnesium must react with the oxygen to get magnesium oxide. The goal of this experiment is to measure the mass of the magnesium, chemically change it to magnesium oxide, and then find the measurement of the magnesium oxide. 3. Methods: Obtain a clean, dry crucible and a wire triangle. Heat the crucible for approximately five minutes over a Bunsen burner. Make sure to heat the crucible to the hottest part of the Bunsen burner flame (above the tip of the inner blue cone of the flame) to ensure that it glows dull red. After five minutes, turn of the burner. Allow the crucible to cool until there is no heat radiating from the crucible when cupping hands around it as this will contaminate the crucible. Once cooled, do not touch the crucible with fingers, use crucible tongs to carry crucible to a scale. Weigh the crucible to the nearest 0.001g and record. Repeat the process of heating, cooling and weighing the crucible until mass of the crucible and the cover agrees within 0.003 g between weigh ins. Obtain and place approximately 0.3g of magnesium ribbon in the crucible and reweigh the crucible and cover (with the magnesium ribbon) to the nearest 0.001g and record. Make sure that the magnesium is coiled so that it lies flat in the bottom of crucible. Place covered crucible back on ring stand tilting the cover of the crucible slightly to allow oxygen in to form oxide. Heat it gently at first and then increase the heat, and heat the crucible for ten minutes using the hottest part of flame. Turn off the Bunsen burner and allow the crucible to cool down to room temperature. If the magnesium remains in its original shape, then the magnesium was coated with magnesium oxide. Should this happen, Crush the residue with a stirring rod and wash leftover residue with five drops of distilled water. The water will convert magnesium nitride (this came from magnesium hydroxide which will be converted to magnesium oxide during heating). Heat again for 5 minutes. If the residue did not turn gray or white, cool the crucible, add 5 more drops of water and reheat again. Cover the crucible again and allow it to cool to room temperature. Weigh crucible to nearest 0.001 g. Tilt lid of crucible and reheat crucible with the residue for five more minutes. Make sure that the crucible is cooled to room temperature and reweigh to the nearest 0.001g a second time. If the masses differ by more than 0.003 g, reheat again for the third time. Once finished with the experiment, wash the crucible. 4. Results:

Mass of Crucible & Cover (Final)| 32.423 g |
Mass of Crucible, Cover & Mg| 32.679 g |
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