Chemistry
Unit 4: Stoichiometry - Introduction
Name: ______________________
Ms. Boon
Period: ______ Date: ____________
Instructions: Read the article. Highlight or underline key points. Answer the questions on a separate sheet of paper.
Stoichiometry: Let's start with how to say this word. Five syllables: STOY-KEE-AHM-EH-TREE. It's a big word that describes a
simple idea. Stoichiometry is the part of chemistry that studies amounts of substances that are involved in reactions. You might be
looking at the amounts of substances before the reaction. You might be looking at the amount of material that is produced by the
reaction. Stoichiometry is all about the numbers.
All reactions are dependent on the amount of materials – reactants and products. Stoichiometry helps you figure out how much of a
compound you will need, or maybe how much you started with. We want to take the time to explain that reactions depend on the
compounds involved and how much of each compound is needed.
Question 1: What is the purpose of stoichiometry?
What do you measure? It could be anything. When you're doing problems in stoichiometry,
you might look at...
- Mass of Reactants (chemicals before the reaction)
- Mass of Products (chemicals after the reaction)
- Chemical Equations
- Molecular Weights of Reactants and Products
- Formulas of Various Compounds
Now, an example. Let's start with something simple like sodium chloride (NaCl). You start with two
ions and wind up with an ionic/electrovalent compound. When you look at the equation, you see
that it takes one sodium ion (Na+) to combine with one chlorine ion (Cl-) to make the salt. When you
use stoichiometry, you can determine amounts of substances needed to fulfill the requirements of
the reaction. Stoichiometry will tell you that, if you have ten million atoms of sodium and only one
atom of chlorine, you can only make one compound of sodium chloride. It's like this:
10,000,000 Na + 1 Cl --> NaCl + 9,999,999 Na
Question 2: Using the logic described above, how many compounds of MgF2 can be made from 100 atoms of Magnesium and 1
atom of Fluorine?
Let's bump it up a level. When you mix hydrogen gas (H2) and oxygen gas (O2), nothing much happens. When you add a spark to
the mixture, all of the molecules combine and eventually form water (H2O). You would write it like this:
2H2 + O2 --> 2H2O
What does stoichiometry look at here? First, look at the equation. Four hydrogen atoms and two oxygen
atoms are on each side of the equation. It's an important idea to see that you need twice as many
hydrogen atoms as you do oxygen atoms. The number of atoms in the equation will help you figure out
how much of each substance you will need to make the reaction happen. If you make this an extreme
example and fill a sealed container with one million hydrogen molecules and only one oxygen molecule,
the spark won't make an explosion. There is no monster reaction to be created when there is only one oxygen molecule around.
You will make two water molecules and be done.
Question 3: In the water example described above, why are two water molecules formed?
Question 4: If the reactants are 4 hydrogen molecules and 2 oxygen molecules, how many water molecules are produced? How
do you know?
What is a mole? Because atoms are extremely small, scientists use a unit called the mole to measure amounts of atoms.
Simply put, the mole represents a number. Just as the term dozen refers to the number twelve, the mole represents the number 6.02 x
1023. Now that's a big number! While a dozen eggs will make a nice omelet, a mole of eggs will fill all of the oceans on earth more than
30 million times over. Think about it: It would take 10 billion chickens laying 10 eggs per day more than 10 billion years to lay a mole of
eggs. So why would we ever use such a big number? Certainly the local donut store is not going to "supersize" your dozen by giving
you a mole of jelly-filled treats.
Chemistry
Unit 4: Stoichiometry - Introduction
Name: ______________________
Ms. Boon
Period: ______ Date: ____________
The mole is used when we're talking about numbers of atoms and molecules. Atoms and molecules are very tiny things. A drop of
water the size of the period at the end of this sentence would contain 10 trillion water molecules. Instead of talking about trillions and
quadrillions of molecules (and more), it's much simpler to use the mole.
Question 5: How many atoms are in a mole of gold?
Question 6: How many moles of water are there in 6.02 x 1023 molecules of water?
History of the Mole: The number of objects in one mole, that is, 6.02 x 1023, is commonly referred to as
Avogadro's number. Amadeo Avogadro was an Italian physics professor who proposed in 1811 that equal
volumes of different gases at the same temperature contain equal numbers of molecules. About fifty years later, an Italian scientist
named Stanislao Cannizzaro used Avogadro's hypothesis to develop a set of atomic weights for the known elements by comparing the
masses of equal volumes of gas. Building on this work, an Austrian high school teacher named Johann Josef Loschmidt calculated the
size of a molecule of air in 1865, and thus developed an estimate for the number of molecules in a given volume of air. While these
early estimates have since been refined, they led to the concept of the mole - that is, the theory that in a defined mass of an element
(its atomic weight) there is a precise number of atoms: Avogadro's number.
Question 7: What is the mole concept? What is Avogadro’s number?
Molar Mass: A sample of any element with a mass equal to that element's atomic weight (in grams) will contain precisely one mole of
atoms (6.02 x 1023 atoms). For example, helium has an atomic weight of 4.00. Therefore, 4.00 grams of helium will contain one mole
of helium atoms. You can also work with fractions (or multiples) of moles:
Other atomic weights are listed on the periodic table. For each
element listed, measuring out a quantity of the element equal to its
atomic weight in grams will yield 6.02 x 1023 atoms of that element.
Mole/Weight Relationship Examples Using Helium
Moles Helium
# Helium Atoms
Grams Helium
1/4
1.505 x 1023
1g
Question 8: Use your periodic table to find the molar masses of
carbon, calcium, cesium, chlorine and chromium.
1/2
3.01 x 1023
2g
1
6.02 x 1023
4g
The atomic weight of an element identifies both the mass of one mole
of that element and the total number of protons and neutrons in an
atom of that element. How can that be? Let's look at hydrogen. One
mole of hydrogen atoms will weigh 1.01 grams.
2
1.204 x 1024
8g
10
6.02 x 1024
40 g
A Hydrogen Atom
Each hydrogen atom consists of one proton surrounded by one electron. But remember, the electron weighs so
little that it does not contribute much to an atom's weight. Ignoring the weight of hydrogen's electrons, we can
say that one mole of protons (H nuclei) weighs approximately one gram. Since protons and neutrons have about
the same mass, a mole of either of these particles will weigh about one gram. For example, in one mole of
helium, there are two moles of protons and two moles of neutrons - four grams of particles.
Molecular Weight: If you stand on a scale with a friend, the scale will register the combined weight of both you and your friend. When
atoms form molecules, the atoms bond together, and the molecule's weight is the combined weight of all of its parts. For example,
every water molecule (H2O) has two atoms of hydrogen and one atom of oxygen. One mole of water molecules will contain two moles
of hydrogen and one mole of oxygen. A bottle filled with exactly 18.02 g water will contain 6.02 x 1023 water molecules. The concept of
fractions and multiples described above also applies to molecules: 9.01 g of water would contain 1/2 mole, or 3.01 x 1023
molecules. You can calculate the molecular weight of any compound simply by summing the weights of atoms that make up
that compound.
Question 9: Use your periodic table to find the molecular weight of the following compounds: (a) methane: CH4, (b) ammonia: NH3, (c)
carbon dioxide: CO2, (d) hydrochloric acid, HCl