Empirical Form of Magnesium Oxide In the theory discussion, it was suggested tha
ID: 1006480 • Letter: E
Question
Empirical Form of Magnesium Oxide
In the theory discussion, it was suggested that the magnesium could be weighed directly. However, in the actual experimental procedure, the mass of the magnesium was determined by an indirect measurement. Explain the advantages of using an indirect measurement to determine the mass of magnesium used. When magnesium nitride is formed, nitrogen has combined with the magnesium. Where does nitrogen go after the reaction is completed? Comparing your class data for the experiment, would you expect the results (mole ratio) to be the same from student to student? Defend your answer. Should the mole ratio vary depending on the amount of magnesium used? Defend your answer. A student doing this experiment used magnesium that had a dull white coating. What is the chemical identity of the dull white coating? What effect would this dull white coating have on the amount of magnesium? A student wanting to leave lab early did not wait for the crucible, cover, and contents to cool down completely. What would be the effect on the amount of magnesium measured? The amount of oxygen? What would happen to the mole ratio?Explanation / Answer
Answer 2)
Air contains not only oxygen gas but also a large proportion of nitrogen gas. Although nitrogen gas is not particularly reactive, magnesium is a very reactive metal, and, at the temperatures achieved during the combustion of magnesium some magnesium will also react with nitrogen gas to give magnesium nitride.
This magnesium nitride can be converted to magnesium oxide by adding few drops of water. When the combustion reaction is completed, and the crucible is cool; add a few drops of water to wet the entire sample to convert magnesium nitride to magnesium hydroxide and ammonia gas as follows,
Mg3N2 + 6H2O 3Mg(OH)2 + 2NH3
Heat the crucible very gently until the product appears to be dry, then heat it strongly to remove excess water:
Mg(OH)2 MgO + H2O
Answer 5)
Aim of the experiment is to convert Mg to MgO and find out its empirical formula and to do so we have to weight Mg ribbon accurately. Since magnesium is very reactive metal it forms magnesium oxide on its surface when get exposed to air, which is dull white in colour. And if some of Mg is already converted to MgO then we would get false weight of magnesium ribbon and in turn we get wrong results (accurate weight of Mg cannot be calculated). Hence it is advisable to remove white coating of Mg ribbon before weighing.
Answer 6)
If student does not wait till the crucible, lid and inner contents get cool down before weighing they may get false reading of weight because. When content is still hot opening of the lid allows white ashes (MgO) to escape from the crucible (systematic error). This will affect the final mass taken which determines the mass of oxygen reacted with the magnesium. This loss of white MgO fumes gives large difference between mass of the product (MgO) and mass of the magnesium weight in starting. Therefore this big difference would seem mass of the oxygen and would give large ratio for Mg/O.
Answer 3)
If all students are carefully following all instructions while weighing, heating and cooling of Mg ribbon then they are expected to get same mole ratio.
Answer 4)
Mole ratio does not change or vary depending upon weight of magnesium used. Mole ration is basic identity of compound it depends upon atomic number, no. of valance electrons in elements and since this doesn’t change with weight of metal, mole ratio would also stay constant.
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