Vocabulary: Avogadro’s number, balanced equation, cancel, coefficient, conversion factor, dimensional analysis, formula mass, molar mass, mole, molecular mass, stoichiometry







Exploration: Stoichiometry


Directions: Follow the
instructions to go through the simulation. Respond to the questions and prompts
in the orange boxes.


Vocabulary: Avogadro’s number, balanced equation, cancel, coefficient, conversion
factor, dimensional analysis, formula mass, molar mass, mole, molecular mass,


Prior Knowledge Questions (Do these BEFORE
using the Gizmo.)


A 250 mL glass of
orange juice contains 22 grams of sugar. How much sugar is in a two-liter
(2,000 mL)

of orange juice?



It requires two
sticks of butter to make a batch of 20 cookies. How much butter will it take to
make 150




Gizmo Warm-up

Just as a cook
follows a recipe to decide how much of each ingredient to add, a chemist uses stoichiometry to determine the amounts of
substances involved in chemical reactions. The Stoichiometry Gizmo allows you to try your hand at figuring out the
amounts of reactants and products that take part in a chemical reaction.


To begin, check
that this equation is shown:


Fe2O3 + 3CO ? 2Fe + 3CO2


1.     Look at the coefficients
(such as the “3” in 3CO) in front of each substance in the equation. The
coefficients tell you how many molecules or atoms take part in a chemical
reaction. In the spaces below, list the number of each molecule or atom in the











In a balanced equation,
the same number of each kind of atom is shown on each side of the equation.
Calculate the number of iron (Fe), oxygen (O), and carbon atoms (C).

















Based on these
values, is the equation balanced?




Activity A:

Get the Gizmo ready:
●    Check that the equation is still:
Fe2O3 + 3CO ? 2Fe + 3CO2
●    If not, click New equation
until it reappears.


Introduction: A mole is:

A mammal known for digging up gardens.

A small, dark spot on the skin.

A spy embedded within an enemy government.

6.02 × 1023 particles of a substance.

All of the above.

The correct
answer, of course, is E. In chemistry, the mole (mol) is defined as an amount
of a substance that contains 6.022 × 1023 particles of that
substance. This number, called Avogadro’s number,
is special because this number of particles has a mass in grams that is equal
to the mass (in unified mass units) of a single particle of the substance.


Question: How do scientists find the formula mass and molar mass of a

 1.    Calculate: The formula mass of a compound is the sum
of the masses of each atom in the chemical formula. The unit of formula mass is
the unified mass unit (u). Formula mass is also called molecular mass if the
compound is composed of molecules.

Iron’s atomic mass is 55.85 u, carbon’s mass is 12.01 u, and oxygen’s
mass is 16.00 u.


Calculate the formula mass of carbon monoxide
(CO) by adding the atomic mass of carbon and

the atomic mass of oxygen:





Calculate the formula mass of carbon dioxide



Calculate the
formula mass of iron (III) oxide (Fe2O3):


Infer: A mole of a substance has a mass in grams that is equal to the formula
mass. For example, a carbon atom has an average mass of 12.01 u. A mole of
carbon has a mass of 12.01 g. Based on their formula masses, list the molar
mass of each substance. The unit for molar mass is g/mol, or grams
per mole.










Check your answers on the Gizmo by inspecting the middle row of tiles on
the right side of the Gizmo. These tiles show the units “1 mol” on top and “g”


Practice: Hydrogen has an atomic mass of 1.01 u. What is the molar mass of these
substances? (Remember to use the units g/mol.)











Activity B:
Canceling units

Get the Gizmo ready:
●    Check that the equation is still:
Fe2O3 + 3CO ? 2Fe + 3CO2


Introduction: While solving problems in stoichiometry, it is
useful to pay attention to the units of the answer. The process of comparing
units is called dimensional analysis. A common
technique involves using conversion factors
to convert from one unit to another. Units that appear in the numerator and
denominator of a fraction can be canceled out.
For example, converting 2 moles of carbon monoxide to grams involves
multiplying by a conversion factor:


2 mol CO • 28.01 g CO = 56.02 g CO

                                                                     1 mol CO


All conversion factors are equivalent to one.
For example, the conversion factor given above is equivalent to one because the
numerator (28.01 g CO) and denominator (1 mol CO) represent the same amount of
CO. The “mol CO” unit is canceled, leaving an answer unit of grams.


Question: How do we solve problems in stoichiometry?


Observe: The first question is: “How many moles of carbon monoxide (CO) are
required to react completely with 1.75 moles of iron (III) oxide (Fe2O3)?”
(If this is not the question you see, click New question until it appears.)



What unit is given in the question?



What quantity is asked for?



Find: Look for the conversion factor that contains the units “mol Fe2O3”
on top and “mol CO” on the bottom. Drag the tile containing this factor down to
the green strip at the bottom.


to the tile, how many moles of CO react with one mole of Fe2O3?



Analyze: To get an answer in moles of CO, you need to cancel the moles of Fe2O3.
Turn on Show units.


units are given to the right of the equals sign?



B.      If these aren’t the units
you want, click Flip tile. What unit
is given now?




Calculate: If the units are correct, multiply or divide the numbers to solve the



How many moles of CO will react with 1.75
moles of Fe2O3?



Turn on Show
numerical result. Were you correct?







Practice: Turn off Show units and Show numerical result. Click New question, and use what you’ve
learned to solve another stoichiometry problem. For each problem, list the
units given, the units asked for, and the solution.


The problems in
the Gizmo are given in random order, so you may have to click Next question several times to see a
new problem. (Note: Each term in the equation is either a solid (s), liquid
(l), gas (g), or an aqueous solution (aq).)


If you are stuck,
try one of the following hints:

If the given unit is grams (or liters or
particles), convert from that unit to moles first. Then convert to moles of the
answer substance.

If the answer unit is grams, liters, or
particles, find the number of moles of the answer substance first. Then convert
the moles of answer substance to the desired unit.

If you have a calculator, try to calculate the
solution to each problem yourself before turning on the Show numerical solution checkbox.




What volume of carbon dioxide (CO2)
will be produced if 2.90 moles of iron (Fe) is produced? [Note: In the Gizmo,
it is assumed that all gases are at standard temperature and pressure, or STP.]









What mass of iron (Fe) can be obtained from 3.80
g iron (III) oxide (Fe2O3) reacting with excess carbon
monoxide (CO)?


Given unit:


Answer unit:





How many moles of iron (Fe) will be produced
from 6.20 moles of carbon monoxide (CO) reacting with excess iron (III) oxide


Given unit:


Answer unit:





How many molecules of carbon monoxide (CO) are
needed to react with excess iron (III) oxide (Fe2O3) to
produce 11.6 g of iron (Fe)?


Given unit:


Answer unit:





On your own: Click New equation to try
solving problems with a new chemical equation. There are five equations in all,
and five problems per equation. The Gizmo will keep track of how many problems
you solve. Good luck!

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