of Magnesium Compounds and the Stoichiometry of Reactions
Introduction to Experiment:
1) Magnesium metal reacts with oxygen and chloride (from hydrochloric acid) to form magnesium salts.
a. Mg(s) + O2(g) ( magnesium oxide salt
b. Mg(s) + HCl(aq) ( magnesium chloride salt + byproduct.
2) From determining the mass of reactant magnesium metal and the mass of magnesium salt product, the mass of oxygen and chloride can be determined.
3) Once the mass of reactants is determined, these amounts can be converted to moles.
a. The ratio between the number of moles of magnesium used and the number of moles of oxygen or chlorine bound can be calculated. This will provide the empirical formula.
Introduction to Empirical Formula:
1) The empirical formula is the formula of a substance written with the smallest integer subscripts (whole numbers).
2) It is the simplest of formulas that is directly related to the percent composition.
3) For most ionic compounds, the empirical formula is the one reported for the compound.
a. Examples:
i. LiCl (1:1 ratio of elements)
ii. CaF2 (1:2 ratio of elements)
iii. CrO3 (1:3 ratio of elements)
iv. Na2S (2:1 ratio of elements)
Introduction to Stoichiometry:
1) Stoichiometry is the calculation of the quantities of reactants and products involved in a chemical reaction.
a. It is based on the balanced chemical equation and on the relationship between mass and moles.
2) The balanced chemical equation can be interpreted in numbers of molecules, but generally chemists interpret equations as “mole-to-mole” relationships.
3) For example, the Haber process for producing ammonia involves the reaction of hydrogen and nitrogen.
a. The balanced equation: N2(g) + 3H2(g) ( 2NH3(g)
b. This balanced chemical equation shows that 1 mole of N2 reacts with 3 moles of H2 to produce 2 moles of NH3.
c. Because moles can be converted into mass (using the molar mass or molecular weight), a mass interpretation of the chemical equation can be performed.
Substances to include in the Table of Chemical and Physical Properties:
The molar mass of the following substances: Magnesium, hydrochloric acid, oxygen gas, magnesium chloride, magnesium oxide.
Part 1
Procedure for Reacting Magnesium Metal with Oxygen:
1) Obtain one crucible and a lid. Clean and dry if necessary.
a. Record an initial mass of the crucible WITH lid.
2) Heat the empty crucible on a hot plate with the lid slightly ajar for 5 minutes to drive away adsorbed water.
a. Be careful to handle only with crucible tongs when hot!
3) Allow to cool to room temperature.
4) Record the mass of the cooled crucible with lid.
5) Place crucible back onto the setup and reheat the crucible for 2 minutes.
a. Allow to cool to room temperature and determine mass.
b. If there has not been a significant change in mass, record the mass and continue on with the experiment.
i. A significant change in mass can include a mass difference of ≥ 0.01g.
c. If there has been significant change in mass, reheat (for 2 minutes) until a constant mass is obtained. Record the constant mass.
6) Obtain one 6 cm ribbon of pure magnesium metal which has a mass of ~0.05g. If tarnished, use sandpaper to polish to a shine.
7) Create a loose roll, place into the crucible, and obtain the mass of this strip of magnesium metal on an analytical balance.
a. It would be advisable to place the crucible + lid of constant mass on the balance and tare it (set it equal to zero) and then weigh the sample into the crucible.
b. Record the mass of the magnesium metal strip.
8) Place crucible with your magnesium back onto the setup with the lid slightly ajar.
9) If heated strongly enough, the magnesium should ignite and white smoke should appear.
a. Do not look directly into the burning magnesium flame!
b. Cover the crucible temporarily using the crucible tongs to hand the lid.
c. Once the smoke subsides, allow the lid to sit slightly ajar for the remainder of the experiment.
10) Next, allow the hot plate (on high setting) to heat the crucible with the lid slightly ajar for at least 10 minutes.
a. The bottom of the crucible should glow red if enough heat is applied.
b. After 10 minutes, remove the lid with the crucible tongs to check if all magnesium metal has been burned.
11) Once all magnesium metal has burned, remove the heat and place the crucible and lid on a ceramic square to cool.
12) Once cool, deliver 5 drops of deionized water into the crucible, place back onto the heating apparatus, and heat the crucible with the lid slightly ajar for at least 5 minutes.
13) After the allotted time, remove the heat and place the crucible and lid on a ceramic square to cool.
14) Once crucible is cool, record the mass of the crucible with lid containing the oxide.
Clean – Up:
15) Place all oxide product into the trash can.
16) Make sure that all gas valves are turned off!
Sample Data Table for Magnesium Metal with Oxygen:
a) Initial Mass of Crucible + Lid in grams: ________
b) Mass of Crucible + Lid After First Heating in grams: ________
c) Mass of Crucible + Lid After Second Heating in grams: ________
d) Stable Mass of Crucible + Lid in grams: ________
e) Mass of Magnesium Metal in grams: ________
f) Moles of Magnesium Metal (e/Mm of Mg): ________
g) Mass of Crucible + Lid + Oxide product in grams: ________
h) Mass of Oxide Product in grams: (g – d): ________
i) Mass of Oxygen/Oxide in grams (h – e): ________
j) Moles of Oxygen/Oxide (i/Mm of O): ________
k) Experimental Mole to Mole Ratio of Mg to O*: ____:_____
l) Experimental Empirical Formula for Oxide Product: ________
m) True Mole to Mole Ratio of Mg to O: ____:_____
n) True Empirical Formula for Oxide Product: ________
o) Balanced Equation for the Reaction: _____________________________
Yield Information:
Actual Yield for Magnesium Oxide in grams: _________
Theoretical Yield for Magnesium Oxide in grams: _________
Percent Yield for Magnesium Oxide: _________
*To determine the mole to mole ratio in whole numbers, divide the number of moles for both Mg and O by the smallest number of moles from the two. Round values to the nearest whole number.
Part 2
Procedure for Reacting Magnesium Metal with HCl: (Please carry out this experiment in fume hood)
1) Obtain one evaporating dish. Clean and dry if necessary.
a. Record an initial mass of the evaporating dish.
2) Heat the empty evaporating dish using hot plate (on medium setting) for 5 minutes to drive away adsorbed water.
a. Be careful to handle only with crucible tongs when hot!
3) Allow to cool to room temperature.
4) Record the mass of the cooled evaporating dish.
5) Place evaporating dish back onto the hot plate and reheat the crucible for 2 minutes.
a. Allow to cool to room temperature and determine mass.
b. If there has not been a significant change in mass, record the mass and continue on with the experiment.
i. A significant change in mass can include a mass difference of ≥ 0.01g.
c. If there has been significant change in mass, reheat (for 2 minutes) until a constant mass is obtained. Record the constant mass.
d. Obtain the mass of the evaporating dish and watch glass.
6) Obtain one 6 cm ribbon of pure magnesium metal which has a mass of ~0.05g. If tarnished, use sandpaper to polish to a shine.
7) Create a loose roll, place into the evaporating dish, and obtain the mass of this strip of magnesium metal on an analytical balance.
a. It would be advisable to place the dish of constant mass on the balance and tare it (set it equal to zero) and then weigh the sample into the dish.
b. Record the mass of the magnesium metal strip.
8) Measure out at least 1.0 mL of 6M hydrochloric acid (HCl).
a. Be extremely careful with this acid; it is corrosive!
9) Before adding the acid to the evaporating dish, line the benchtop with paper towels and place evaporating dish with magnesium inside onto the paper towels.
10) Slowly pour the acid overtop of the magnesium metal in the evaporating dish.
a. Record observations of the process that occurs.
b. Do not inhale vapors.
11) Once all of the magnesium has reacted, place onto the apparatus shown below UNDER THE FUME HOOD.
12) Cover the dish with a watch glass.
13) Heat the solution using hot plate until dryness.
a. Notice that unreacted hydrochloric acid and water will steam off.
b. Use crucible tongs to carefully remove the watch glass, and place into a paper towel.
14) Once a dry powder is observed, place evaporating dish onto a cooling square, allow for cooling, and then obtaining the mass of the powder product.
Clean – Up:
15) Place all chloride products into the labeled waste bottle.
16) Make sure that all gas valves are turned off!
Sample Data Table for Magnesium Metal with HCl:
a) Initial Mass of Evaporating Dish in grams: ________
b) Mass of Evaporating Dish After First Heating in grams: ________
c) Mass of Evaporating Dish After Second Heating in grams: ________
d) Mass of Evaporating Dish + Watch Glass in grams: ________
e) Mass of Magnesium Metal in grams: ________
f) Moles of Magnesium Metal (e/Mm of Mg): ________
g) Mass of Evaporating Dish + Watch Glass +Chloride Product: ________
h) Mass of Chloride Product in grams: (g – d): ________
i) Mass of Chlorine/Chloride in grams (h – e): ________
j) Moles of Chlorine/Chloride (i/Mm of Cl): ________
k) Experimental Mole to Mole Ratio of Mg to Cl*: ____:_____
l) Experimental Empirical Formula for Chloride Product: ________
m) True Mole to Mole Ratio of Mg to Cl: ____:_____
n) True Empirical Formula for Chloride Product: ________
o) Balanced Equation for the Reaction: _____________________________
Yield Information:
Actual Yield for Magnesium Chloride in grams: _________
Theoretical Yield for Magnesium Chloride in grams: _________
Percent Yield for Magnesium Chloride: _________
*To determine the mole to mole ratio in whole numbers, divide the number of moles for both Mg and Cl by the smallest number of moles from the two. Round values to the nearest whole number.
Banerjee
General Chemistry-I Lab
PAGE
6
General Chemistry – I Lab Banerjee
—-
Magnesium Compounds and Reaction Stoichiometry
Experimentation Overview:
1) Magnesium salts are formed when magnesium metal combines with oxygen and chloride (from hydrochloric acid).
Mg(s) + O2(g) (magnesium oxide salt).
Mg(s) + HCl(aq) (magnesium chloride salt + byproduct).
2) The mass of oxygen and chloride can be calculated by determining the mass of reactant magnesium metal and the mass of magnesium salt product.
3) Once the mass of the reactants has been calculated, the quantities can be converted to moles.
a. A ratio can be determined between the number of moles of magnesium utilized and the number of moles of oxygen or chlorine bound. The empirical formula will be provided as a result of this.
An Overview of the Empirical Formula:
1) An empirical formula is a substance formula written with
