Purpose

Purpose:
The quantitative relationship of mass and amount will be studied using the combustion reaction of magnesium metal to determine the empirical formula of magnesium within two crucibles. Each crucible went through the process of firing to get rid of any excess water or substances left over at the bottom of the crucible. In the presence of air, when magnesium is heated, magnesium metal is oxidized by oxygen gas to magnesium oxide. The reaction is to allow some Mg to react with gaseous N2; in addition, there’s a higher percentage of N2 gas in the atmosphere than O2, but O2 is more responsive and the magnesium oxide is formed in a greater amount than the nitride. By adding additional water, the small amount of nitride that was formed can be removed, which will convert the nitride to magnesium hydroxide and ammonia gas. The second heating with the magnesium, water, and oxygen causes the loss of water and conversion of the hydroxide to the oxide. The masses are used to calculate the empirical formula of magnesium oxide, which is then compared to the expected empirical formula. As this was done, Mass by Difference was used to determine the initial mass of magnesium, which lead to the rest of the calculations to find the ratio of Mg to O.

Procedure:
The procedure can be found in: CHE 131 Experiment 4, General Chemistry I Lab, Autumn Quarter 2018-2019, DePaul University. Online https://www.d2l.depaul.edu (accessed on October 10, 2018).
There were no deviations from the procedure cited above.

Data and Results:
To solve for the empirical formula, Mass by Difference was used to determine the initial mass of magnesium, which lead to the rest of the calculations to find the ratio of Mg to O. The resulting masses are used to calculate the empirical formula of magnesium oxide, which is then compared to the expected empirical formula..

Table 1: Masses of magnesium, magnesium oxide, and oxygen to determine the Mg to O ratio
Crucibles
Empty Constant Mass (g)
Crucible + Mg Mass (g)
Crucible + MgO after 1st Firing Mass (g)
Crucible + MgO after 2nd Firing Mass (g)
Crucible + MgO after 3rd Firing Mass (g)
Mg:O
1
35.9599
36.3342
36.5573
36.5781
36.5614
1.08:1
2
35.3657
35.6155
35.7648
35.7714
35.7667

To solve for the initial mass of magnesium, mMg, the formula used to calculate was the Mass by Difference formula. The mass of the crucible was subtracted from the mass of the crucible with the magnesium.

mMg=mc + Mg-mc
mMg=36.3342 g-35.9599 g
mMg=0.3743 g
To find the number of moles of magnesium, the mass of magnesium is divided by the molar mass of magnesium, which is 24.3050 gmol-1.
nMg=mMgmolar mass of Mg
nMg=0.3743 g24.3050 gmol-1
nMg=0.0154 mol-1
To find the mass of the oxygen, the mass of the crucible and magnesium would be subtracted from the mass of the crucible and magnesium oxide. This will help find the mass by canceling out the crucible and magnesium.
mO=mc + Mg + O-mc + Mg
mO=36.5614 g-36.3342 g
mO=0.2272 g
To find the number of moles of oxygen, the mass of oxygen is divided by the molar mass of oxygen, which is 15.9994 gmol-1.
nO=mOmolar mass of O
nO=0.2272 g15.9994 gmol-1
nO=0.0142 mol-1
To determine the molar ratio of magnesium, Mg, it would be divided by the lowest moles, which is oxygen.
Mg=nMgnO
Mg=0.0154 mol-10.0142 mol-1
Mg=1.08
To determine the molar ratio of oxygen, O, it would be divided by the lowest moles, which is oxygen.
O=nOnO
O=0.0142 mol-10.0142 mol-1
O=1
The empirical formula for MgO is Mg11O10because it was close to to being a 1:1 ratio.

Discussion:
In order to obtain magnesium oxide, the magnesium has to be heated in a crucible. Magnesium reacts quicker when oxygen is exposed to the magnesium and this process creates a white light, which means that MgO is created. The magnesium starts to look dull and has a texture of ashes. The reaction is to allow some Mg to react with gaseous N2; in addition, there’s a higher percentage of N2 gas in the atmosphere than O2, but O2 is more responsive and the magnesium oxide is formed in a greater amount than the nitride. By adding additional water, the small amount of nitride that was formed can be removed, which will convert the nitride to magnesium hydroxide and ammonia gas. The second heating with the magnesium, water, and oxygen causes the loss of water and conversion of the hydroxide to the oxide.

In this experiment, a error is the not enough supply of oxygen. The experiment allowed oxygen to enter from a slight opening, which could not be enough oxygen to react with magnesium to create magnesium oxide. This error could have lowered the mass and moles of oxygen to make it a unreliable empirical formula. This could be reduced by allowing more oxygen to enter the crucible with the magnesium. Another error is properly not getting rid of the excess water or substance left over at the bottom of the crucible when it was going through the process of firing in the beginning.

The empirical formula determined from the experimentally doesn’t agree with the expected formula based on the periodic table because it wasn’t a one to one ratio. This could’ve happened because not enough oxygen was supplied into the crucible containing the magnesium. If there were unreacted magnesium metal left in the crucible at the end of the experiment, the molar ratio of Mg to O would be too high in Mg because not enough magnesium was burned, which meant that there would be a lot more oxygen in the crucible rather than magnesium. If the experimental empirical formula was Mg2O3, the implied charges on the ions in Mg2O3 would have a negative charge on the oxygen and a positive charge on the Mg. The charges agree with those expected from trends in the periodic table because Mg is in the group 2 ion, which as a positive two charge, and O is in the group 6 ion, which as a negative two charge.