John Dalton’s atomic theory states that elements combine in simple numerical ratios to form compounds. A compound, no matter how it is formed, always contains the same elements in the same proportion by weight. The law of mass conservation states that mass can neither be created nor destroyed. In this experiment, the mass of the metal was not destroyed or created; the metal’s mass was simply changed into a compound form once the gas reacted to it.
The net mass of the reactant side of the equation and the product side of the equation should be equal if the experiment is done correctly. The molecular formula represents the number of all elements in a compound. The empirical is the simplest whole number ratio of the elements in that compound.
Combustion reactions always involve oxygen and are almost always exothermic. Exothermic reactions give off energy in heat form. The purpose of this experiment is to find the empirical formula of a compound using whole numbers. To investigate this experiment, the masses of the metal and gas were measured to obtain the empirical formula of the compound.
Before starting the experiment, the materials needed were gathered: crucible and lid, Bunsen burner, deionized or distilled water, striker, magnesium ribbon, sandpaper (if needed), clay triangle, wire pad, crucible tongs, electronic scale, ring clamp, experiment stand, paper to record data. Two of each necessary material was gathered in order to conduct two trials at once.
To prepare for the experiment, the ring clamp was attached to the stand at about 2/3s the way up the stand. The crucible and lid were rinsed with water, dried, and then placed on a clay triangle. The Bunsen burner was hooked up to the gas line and the gas was turned on. The fire was started with the striker and the flame was adjusted to the height of the ring clamp. The crucible and lid were heated gently for 4-6 minutes until the bottom of the crucible became red. The flame was intensified and the crucible and lid were heated for another 10-12 minutes. The crucible and lid were allowed to
cool on the wire pad. The mass of the cooled crucible and lid was recorded using the electronic scale.
This procedure was repeated once more for each trial. In each trial, the ribbon was placed into the crucible and the lid was placed over it. The mass of the crucible, lid and magnesium was recorded. The crucible containing the magnesium was heated gently for 2-3 minutes. The heat was gradually intensified and heated for another 2-3 minutes. One side of the lid was lifted with the crucible tongs to allow the oxygen inside. The metal started glowing. The crucible, lid and compound were heated for another 3 minutes.
The metal was checked periodically until no more glowing was observed. The crucible was then removed from heat and then cooled on the wire pad. 3 drops of deionized water was added to the cooled compound. The crucible was reheated with the lid partially off, allowing the water vapor to escape. The sample was heated slowly and then the heat was intensified for 15-17 minutes. The crucible, lid and compound were allowed to cool on the wire pad. The mass of the crucible, lid and compound was recorded. The sample was reheated for an extra 5 minutes, then the combined mass of the crucible, lid and metal oxide was measured. The metal oxide was disposed of in the proper marked contained and the crucible was cleaned of any residue and rinsed with deionized water.
After 1st Heating
After 2nd Heating
Mass of Crucible, Lid and Metal
Mass of Metal
Mass of Crucible, Lid and Metal Oxide after 1st heating
2nd Heating Mass Measurement
Mass of Metal Oxide
Number of Moles of Metal in the Compound
Mass of Oxygen in Compound
Number of Moles of Oxygen in the Compound
Simplest Whole Number Ratio of Oxygen to Metal
Empirical Formula for the Compound using Whole Numbers
In order to have magnesium oxide, MgO, the magnesium strip had to be heated. Under normal circumstances, room temperature, magnesium metal, Mg, reacts very slowly with the oxygen, O, in the air. However, as magnesium is heated, it reacts quicker with the oxygen and burns with a white light to produce MgO.
To protect others from the smoke, containing Magnesium Oxide, the crucible had to remain covered. Some magnesium oxide escaped, when the crucible was not covered. The crucible had to be slightly ajar when heating up the magnesium, so that oxygen could get to the reaction. Without oxygen, a fire cannot exist. The shininess of the metal Mg turned to a dull appearance as it changed to MgO. As the magnesium reacted to the oxygen, it also reacted with the nitrogen in the air to form magnesium nitride, Mg3N2.
To expel the nitrogen from the crucible, we added water to the mixture and heated it up. This would cause the Mg3N2, to react with the water, H2O, to form ammonia, NH3, and magnesium hydroxide, Mg(OH)2. The NH3 was driven off during the heating. One sign of this reaction was the ammonia smell given off. This is because upon heating, the Mg(OH)2 would break into MgO and H2O, which would be driven off by the heat. The second reheating was so that any remnants of the Mg(OH)2 of the crucible had been converted to MgO. This was also to have an accurate final mass of our product MgO.
After the lab, the inside of the crucible was black. This is because the magnesium not only reacted with the oxygen and the nitrogen in the air but also with the porcelain of the crucible. The reason for waiting for the crucible to cool before weighing it was because at higher temperatures, the molecules inside are still active, causing the weight to be off. During Trial 2, the magnesium was not properly burned off and caused the calculations to be off. The magnesium looked as if it had stopped glowing, but the inside coil was not completely burned.
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