Quantitative Determination of a Chemical Formula
Purpose: The purpose of this lab is to experimentally determine
the empirical formula of magnesium chloride.
Procedure: Data necessary to determine the empirical formula
includes the mass of each element in a sample of the compound.
Magnesium will be reacted with a solution hydrochloric acid. This
reaction will produce hydrogen gas and leave a solution of
magnesium chloride. To obtain the mass of magnesium, mass an
empty test tube and then mass of the test tube with the
magnesium to be reacted in it. Record both and subtract to
calculate the mass of the magnesium sample.
The reaction should be conducted carefully adding a solution
of 1M HCl a few drops at a time until the magnesium is completely
“dissolved.” While the reaction is underway, test the gas
produced with a burning wood splint. A popping sound will indicate
the presence of hydrogen. If there is still magnesium visible
after the gas production stops, continue adding acid a few drops
at a time. Once the magnesium is gone do not add any more acid.
Place the test tube in a boiling water bath to evaporate the
remaining water, in order to leave the solid magnesium chloride.
The remaining solid product can be gently heated with a
bunsen burner to ensure any water is driven off. Record the
appearance of the solid. Mass the test tube and solid product.
The mass of the magnesium chloride may be calculated by
subtracting the empty tube mass. The mass or chlorine may be
obtained by subtracting the mass of the magnesium and test
tube.
Results: During the reaction hydrogen gas was released . After
evaporation, the solid appeared tan in color. Using the data
collected, the percent composition was found to be 24%
magnesium and 76% chlorine. The empirical formula was
calculated to be MgCl2.
Error Analysis: Possible errors effecting the results include
adding too much HCL or not enough to completely react the
magnesium. Too much acid will leave some excess in the solid
product. Since the magnesium was massed separately, excess
mass will be included in the mass calculated for the chlorine,
thereby resulting in a higher percent composition for chlorine and
higher ratio of chlorine in the empirical formula. Not enough acid
will cause some magnesium to remain in the product and cause the
mass of chlorine to be calculated low, thereby driving down the
percent composition and ratio of chlorine in the empirical formula.
Other errors could include spillage of the reacting solution, or
chipping the test tube. Any loss of mass will drive down the
results for chlorine. Any contamination, such as wood ash from
the splint test, will drive up the results for chlorine.
Data Table and Calculations
19.88
Mass of test tube and magnesium
Mass of empty test tube
Mass of magnesium
Mass of magnesium chloride and test tube
Mass of magnesium chloride
Mass of chlorine
19.78
0.10
20.2
0.42
0.32
Percent Mass
mass
percent composition
Mg
0.10 / 0.42 x 100 =
24 %
Cl
0.32 / 0.42 x 100 =
76 %
total
0.42
Empirical Formula
Mg
0.10
Cl
24.305
0.32
35.453
Mg
Cl
0.00411438
0.009026034
0.00411438
0.00411438
Mg
Cl
1
2.193777677
Percent Composition based on known formula, MgCl2
mass
number of element in compound
Mg
24.305
1
24.305 / 95.211 x 100 =26 %
Cl
35.453
2
70.906 / 95.211 x 100 =74 %
95.211