Stoichiometry
Limiting and Excess Reactant
Hayeon Jun
Standards
3.3c
A balanced chemical equation represents the conservation of atoms. The
coefficients in a balanced equation can be used to determine mole ratios
in the reaction.
3.3d
The empirical formula of a compound is the simplest whole number ratio
of atoms of the elements in a compound. It may be different from the
molecular formula, which is the actual ratio of atoms in a molecule of
that compound.
3.3f
The percent composition by mass of each element in a compound can be
calculated mathematically.
Objectives
At the completion of the following lessons,
the students will be able to:
- Balance equations given in the formulas for reactants and
products
- Calculate simple mole-mole stoichiometry problems,
given a balanced equation
- Determine the number of moles of a given substance,
given its mass
- Determine the mass of a given number of moles of a
substance
Agenda
1. Aim/Do-Now (5 minutes)
2. Do-Next (2 minutes)
3. Lab/Post-Lab (40 minutes)
4. Exit/Wrap-Up (2-3 minutes)
Lab Report
Aim: What are the limiting and excess
reactants of our lab experiment?
Do-Now
Compare your pre-lab
homework answers
with your lab group.
Discuss among group
members if there are
discrepancies.
Previous Day
Using the chart below, select your group number and the
mass of aluminum metal and copper (II) chloride
dihydrate you will be using in this activity:
Group
1
2
3
4
5
6
Mass of Al (g)
1.00
0.50
0.50
0.50
0.25
0.25
Mass of CuCl2·2H2O (g)
2.00
2.00
4.00
6.00
5.00
4.00
Pre-lab (HW from the night before)
1.
2.
3.
Aluminum metal reacts with aqueous copper (II)
chloride dihydrate to form copper metal, aqueous
aluminum chloride, and water. Write the balanced
equation for this reaction.
What does a solution look like when it contains
dissolved copper (II) ions?
For the balanced reaction, determine the mass of
aluminum that would remain after a 10.00-g Al sample
and 1.00 g of CuCl2·2H2O fully reacted. Show your
work fully.
Aim: What are the limiting and excess
reactants of our lab experiment?
Do-Next
Get into your groups, gather all the materials needed for
the lab, and begin your experiments!
Materials: 250 mL beaker
100 mL graduated cylinder,
spoon or stirring rod,
watch glass,
copper (II) chloride dihydrate,
aluminum foil, water
Lab Safety Precautions
Wear safety goggles during
this experiment.
Avoid skin contact with
copper (II) chloride
dihydrate.
Wash your hands before
leaving the laboratory.
Lab Procedures
1.
2.
3.
4.
Weigh out the copper (II) chloride dihydrate in a 250 mL beaker.
Using a graduated cylinder, measure out exactly 100 mL of water (tap water
is fine) and add it to the beaker containing the copper (II) chloride dihydrate.
Stir until the salt dissolves completely. Note the color of the solution in your
lab notebook.
Weigh out the appropriate mass of aluminum foil, crumple up into a few
small pieces and add it to the copper (II) chloride dihydrate solution. Stir the
solution gently and then allow it to sit undisturbed for 5 – 10 minutes. Write
observations in your lab notebook.
At the end of the period, label your beaker and cover it with a watch glass.
Your teacher will designate an area to store your beakers overnight. During
the next class period, you will be able to confirm the identity of the limiting
reactant.
Post-Lab
1. Indicate your group number.
2. What is the limiting reactant in this reaction?
3. What is the theoretical yield (in grams) of copper metal?
4. What is the theoretical yield (in grams) of aluminum
chloride?
5. What mass of the excess reactant will remain when the
limiting reactant is consumed?
Limiting and Excess Reactants
The limiting reactant is the reactant that is entirely consumed
when a reaction goes to completion.
The excess reactant is the reactant that is not completely
consumed, and is left over.
The moles of the product are always determined by the
starting moles of the limiting reactant.
Lab Report
Include a brief introduction, data, the balanced reaction
equation, and calculated results (including work).
Determine the relative size of any error between your
predicted post-reaction mass of the Al and its actual mass.
Discuss possible causes for this error; be sure that the
causes you discuss would result in the error you
experienced, i.e. the actual post-reaction aluminum mass
being larger or smaller than predicted.