EXPERIMENT 14: Moles Stoichiometry(Limiting Reagent)
The Reaction of Acid with Magnesium
Materials:
Three balloons of uniform size
Erlenmeyer flask (3, 125 ml)
Mg ribbon
1.0 M HCl (150 ml)
Masking tape
Objective: To become familiar with the stoichiometry relationship between moles of reactants and moles of products
in a reaction of an acid with magnesium.
INTRODUCTION
This experiment is an excellent classroom demonstration that the amount of product formed is limited by the
amount of reactants available. If done as a demonstration, the amounts of reactants specified below should be
doubled or tripled. The balanced equation for the reaction tells us that one mole of magnesium and two moles of
hydrogen chloride yield one mole of the salt (magnesium chloride) and one mole of hydrogen gas.
Mg + 2HCl  MgCl2 + H2
Determining the mass of magnesium chloride formed would be more complicated than measuring the quantity
of hydrogen formed. We may easily trap the gas and measure it, and from the resulting volumes, we can find
out if limiting the amount of magnesium or the amount of hydrogen chloride will limit the amount of product
formed.
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PROCEDURE
1. Weigh 0.3, 0.6 and 1.2 g of Mg. Make sure the metal is shiny.
2. Place exactly 50 ml of 1.0 M HCl in each of the three clean 125 ml Erlenmeyer flasks, using a funnel so that
you do not get the necks of the flasks wet with the acid. Fig 1
3. The magnesium metal will react faster if it is cut up into small pieces. Cut each of the three strips of Mg into
three piles of Mg pieces. Carefully place each pile into each balloon. Without allowing the Mg pieces to
accidentally drop into the acid, fit the balloons over the mouth of the three flasks and secure by wrapping
with the masking tape. Make sure there are no open flames nearby.
4. With your finger on the outside of the balloon, carefully push the metal pieces in each balloon into their
respective flasks of acid. In each case, gently swirl the flasks from time to time until no further reaction (no
more bubbles) occurs. Record your observations and data on the report sheet. If the balloon breaks, place the
flask in the fume hood away from flames until the reaction is complete. Ask your instructor about the
disposing of wastes.
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EXPERIMENT 14: Moles Stoichiometry(Limiting Reagent)
The Reaction of Acid with Magnesium
REPORT SHEET
Name ___________________________
Instructor ______________________
Date ____________________________
1. How many moles of HCl were used in each flask?
_________________moles
2. Calculate the moles of Mg used in each case : (show calculation)
a. 0.3 g Mg = ___________________ moles
b. 0.6 g Mg = ___________________ moles
c. 1.2 g Mg = ___________________ moles
3. Was a significant amount of magnesium left unreacted? If so, in which case?
4. Explain why this should have happened.
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EXPERIMENT 14: Moles Stoichiometry(Limiting Reagent)
The Reaction of Acid with Magnesium
Name: ____________________________
Pre-Laboratory Questions and Exercises
Due before lab begins. Answer in the space provided.
1. What safety precautions are cited in this experiment?
2. 3.94 g of magnesium metal was used to produce hydrogen gas. Write the balanced chemical equation for
this reaction. How many moles of hydrogen gas would be produced by the complete reaction of the
magnesium?
2. In your reaction, the magnesium is necessarily the limiting reactant, so that it is completely consumed in
the reaction. The amount of hydrochloric acid added to the reaction was more than enough to consume
all of the magnesium. Calculate the exact amount of 12 M hydrochloric acid in mL necessary to
consume 0.040 grams of magnesium metal.
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EXPERIMENT 14: Moles Stoichiometry(Limiting Reagent)
The Reaction of Acid with Magnesium
Name: ____________________________
Post-Laboratory Questions and Exercises
Due after completing the lab. Answer in the space provided.
1. Why is it necessary to clean the magnesium strip before reacting it with hydrochloric acid?
3. If you hadn’t added quite enough hydrochloric acid to consume all of the magnesium metal, what would
be the effect of this error on the percent yield in your experiment? Would the actual yield, and thus the
percent, increase, decrease, or be unaffected by not adding enough HCl? Explain.
4. How would your experimental formula of magnesium chloride “MgClx” have been affected if your
product was not dried completely before weighing it? Would “x” be too high or two low?
5.
a) How many moles of magnesium atoms are in 150 g of magnesium metal?
b) How many magnesium atoms are in this amount of magnesium?
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