The empirical formula of a compound

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Brent Nash
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1 The empirical formula of a compound Reference: Chapter 1, Section 1.2, pages Please note Aim A full risk assessment should be carried out prior to commencing this experiment. Personal safety equipment should be worn. Chemicals should be disposed of safely and with due regard to any environmental considerations. To determine the empirical formula of the compound which forms between magnesium and oxygen. Introduction The empirical formula of a compound shows the simplest ratio of atoms present. In this experiment we will calculate the empirical formula of the compound magnesium oxide, formed by the combustion of magnesium in the air. We will calculate the change in the mass of magnesium on heating it, and assume that the increase in mass is due to the oxygen that has combined. But when magnesium burns in air, the temperature becomes high enough for it to combine with both the oxygen and the nitrogen in the air. The magnesium therefore forms a mixture of magnesium oxide and magnesium nitride after combustion. So we need to convert the magnesium nitride in this mixture to magnesium oxide, before we can calculate the change in mass. The conversion of magnesium nitride is done in two steps: step 1 add water to convert magnesium nitride to magnesium hydroxide, releasing ammonia, NH 3 step 2 Pre-lab questions heat the magnesium hydroxide to convert it to magnesium oxide. 1 What are the formulas of magnesium nitride and magnesium hydroxide? (Look at Chapter 4, pages , if you are unsure of the formulas of the ions.) 2 Write an equation for the reaction in which magnesium nitride forms when magnesium burns in air. 3 Write an equation for the reaction described in step 1 above. 4 Write an equation for the reaction described in step 2 above. Method 1 Record the mass of a clean, dry crucible with its lid. (Handle the crucible with tongs, not your fingers, to avoid moisture and oil from your fingers being transferred.)

2 2 Use fine sand paper to scrape the oxide coating from the surface of a strip of magnesium ribbon approximately 2 cm length. Cut the ribbon into small pieces, place in the crucible, and weigh the crucible, its lid, and contents. 3 Heat the crucible in a hot flame for 10 minutes, ensuring that the magnesium is exposed to air but that no solid escapes. Lift the lid every 2 minutes to allow oxygen to enter the crucible. After this time the magnesium should have been converted to a white powder. 4 Allow the crucible to cool and then add 10 drops of water to convert the magnesium nitride to magnesium hydroxide. 5 Heat the crucible gently to drive off any water, then strongly for 3 minutes to convert the magnesium hydroxide to magnesium oxide. 6 Allow the crucible to cool. Weigh the crucible, its lid, and the product of the reaction. Results Mass / g ± Mass of crucible + lid Mass of crucible + lid + Mg before heating Mass of crucible + lid + product after heating Qualitative data Before heating During heating After heating Appearance of crucible contents Magnesium metal has a grey shiny appearance Metal burned with a bright white light The metal turned into a grey-white powder

3 Analysis Magnesium, Mg Oxygen, O Mass /g ± M / g mol 1 moles / mol Work out the simplest ratio of magnesium atoms to oxygen atoms and so the empirical formula of magnesium oxide. Conclusion and evaluation The theoretical result for the formula of magnesium oxide is MgO. Compare your result for the empirical formula of magnesium oxide with the theoretical result, MgO. Calculate the % error. Calculate the % error arising from random errors in the experiment and compare this with the figure above. Itemize systematic errors and suggest modifications to the experiment to reduce these. For consideration 1 What can you deduce from the fact that magnesium metal is generally covered in an oxide layer? 2 How could you verify that ammonia gas, NH 3, is released during step 4 of the method? 3 What is meant by heating to constant mass? Explain how this could be applied to this experiment.

4 Evaluation The main significant sources of errors may come from: 1. The opening of the lid allows white ashes (MgO) to escape from the crucible (systematic error) Opening of the lid is required to allow oxygen in the surrounding air to enter the crucible, so as to react with the heated magnesium ribbon to form magnesium oxide. However, white ashes (MgO) escapes from the crucible due to the strong heat, causing the convectional air in the crucible, which blows out the magnesium oxide while opening the lid. This means that the final mass taken which determines the mass of oxygen reacted with the magnesium is less. 2. The Mg weighed is not all pure (systematic error) During sanding of the magnesium ribbon, not all of the MgO coating may have been removed. This would have contributed to a lower actual mass of Mg. 3. The duration of heating the magnesium ribbon (systematic error) From the instructions given, we are told to stop the heating only when the powder no longer glows. However, the given magnesium ribbons (even though the same length) do not have the same mass, hence causes the difference in the reaction time and the amount of oxygen reacted. It is possible that if the reaction was allowed to proceed for a longer time, more of the magnesium would have reacted and therefore the mass of magnesium oxide would be greater. Improvement: 1. Quickly close back the lid after allowing some oxygen to enter the crucible This is because white ashes MgO will escape while opening the lid during heating. Therefore, to minimize the amount of MgO escaping the crucible, it is best to quickly close back the lid after allowing some of the oxygen in the surrounding to enter the crucible, to react with the magnesium ribbon. 2. Sand the Mg ribbon sufficiently to remove all the MgO coating. To ensure that all the MgO coating is removed from the surface of the magnesium ribbon, the magnesium should be sanded until all the grey, dull surface coating is removed and the magnesium ribbon looks shiny in appearance. 3. Allow the reaction to continue for a longer period of time. Allowing the reaction to proceed for a longer period of time would allow more magnesium to react to form the product. In the future, I would allow the reaction to proceed for 30 minutes. More trials would minimize this random error that occurred in the experiment, and hence improve accuracy.

5 Equipment list Chemicals / material magnesium ribbon: approx. 6 cm per group (to allow for repeat trials) fine sand paper Apparatus (per group of students) crucible + lid tongs pipe-clay triangle / gauze stand / tripod Bunsen burner balance accurate to at least two decimal places

EMP I RICAL FORMULA OF MAGNESI U M OXIDE

Experiment 6 Name: 53 EMP I RICAL FORMULA OF MAGNESI U M OXIDE In this experiment, you will synthesize oxide via the reaction pathways summarized in Figure 1. Note that [1] is the main reaction and [2]