Aim:
To determine the empirical formula for magnesium oxide.
Data Collection:
Table 1 Mass of empty crucible, crucible + Mg (roughly 1cm pieces of a 15-cm piece of magnesium ribbon), and crucible +MgxOy. (mass measured with an analytical balance).
The mass of crucible + MgxOy was obtained after a continuous process of heating, letting cool, heating with water enough to immerse the content and letting cool of the Mg.
Mass of empty crucible (g)Mass of Crucible + Mg (g)Mass of Crucible +MgxOy (g)
16.04716.21416.294
Uncertainty of analytical balance = + 0.002
*Note that because we are uncertain if Mg has completely reacted with O2 in the first heating, if all the Mg have reacted with O2 instead of N and if the water added has completely vaporized in the second heating(with water), the data and the following calculations may not be as accurate.
Data Presentation and Procession:
1.mass of crucible + Mg – mass of empty crucible =Mass of Mg
16.214 ±0.002 g
– 16.047± 0.002 g
0.167 ± 0.004 g
2.mass of crucible + MgxOy -mass of crucible +Mg = mass of O
16.294 ±0.002g
– 16.214 ±0.002g
0.0800±0.004
3.mass of Mg =0.167±(0.004/0.167*100%)
=0.167±2 % g
4.mass of O = 0.0800±(0.004/0.0800*100%)
=0.0800±5% g
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5.Mole O = (0.0800±5%g) /(15.999g/mol)
=0.00500 ±5%mol
6.Mole Mg = (0.167±2% g)/(24.305g/mol)
=0.00687±2%mol
7.relative ratio of Mg = 0.00687/0.00500 ±(2 % +5%)mol
=1.37±7%mol
=1.37± (7*1.37/100)
=1.37±0.1
8.Relative ratio of O = 0.00500/0.00500 ±(5%+5%)
=1.00±10%
=1.00±(10*1.00/100)
=1.00±0.1
9.Empirical Formula= Mg1.37±0.1O1.00±0.1
=MgO
Table 2 Mass and relative ratio of Mg and O and calculated empirical formula for magnesium oxide as determined by its relative ratio.
Mass of O± uncertainties (g)Mass of Mg ± uncertainties(g)Number of Mole ± % of uncertainties Relative ratio ± uncertaintiesEmpirical Formula
0.167 ±0.0040.0800± 0.004OMgOMgMgO
0.00500± 5%0.00687± 2%1.00± 0.11.37± 0.1
Evaluation:
Compare to the accepted one to one ratio of the empirical formula for magnesium oxide, our calculated empirical formula Mg1.37±0.1O1.00±0.1 suggests an imbalance ratio with less than one oxygen atom for every magnesium atom. This confidence about the proportion of Mg instead of O is due to the certainty of the mass of Mg (a 15 cm magnesium ribbon), and uncertainties of the mass of O whose value may change because of some sources of error during the reaction with Mg (with and without adding water).
Two possible sources of error that may explain the inadequate presence of oxygen atoms are the incomplete reaction of Mg with O during the first heating (without adding water) and the reaction of Mg with N instead of O during the second heating (adding water).
It is possible that not all of the Mg has reacted with O in the given heating time during the first heating or/and water has not broken all the bonds between Mg and N (N occurs naturally with O during heating) for Mg to react with more O thus reducing the presence of oxygen atoms.
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Our experiment can be improved and sources of error can be eliminated, if we repeat the process of heating the content, letting it to cool and measuring its mass during the first and second heating (with and without adding water) until the mass reaches a constant value. In this way, we can assure that all Mg has completely reacted with O.
