Chapter 8 Notes
8.1 Counting by Weighing
The expensive Jelly Bean Shop.
The store charges by the jelly bean.
People come in and order 500 jelly beans and YOU have to count them.
What is an easier way to find 500 jelly beans?
Find the average mass of a jelly bean. For example…the average mass of the jelly beans
is 5.3grams.
The customer wants 500 jelly beans. Instead of counting each individual jelly bean, how
could you quickly give the customer his 500 jelly beans? Answer in space below.
Think about if we were going to make a batch of cookies and the recipe called for
1 cup of flour and 1 cup of butter and that would make 1 cookie. Now what if I wanted to make
500 cookies. Well I would have to have 500 cups of flour and 500 cups of butter, but that
would take forever to measure out. So instead I figure out the average mass (in grams) of a cup
of four and the average mass (in grams) of a cup of butter and then multiply each ingredients
average mass by 500. (Just like we did for the jelly bean example, only this time we have two
ingredients). Now here is the weird/obvious part. Even though I need 1 cup of both of the
ingredients, the masses would be different because butter has a larger average mass then flour.
So in the end to get an equal amount of butter and flour for my recipe, I had to add different
mass amount of each.
Now lets apply that idea to chemistry!!!
In the BALANCED equation Ba + S
BaS ……….the balanced equation tell me I need
1 thingy of barium and 1 thingy of sulfur to make 1 thingy of barium sulfide.
What is the “thingy”………it’s either the atoms, or it can be the molecules/compounds, or it
can be the particles, or it can be the moles.
What is a mole????????
8.3 The Mole
In chapter 4 we explained the concept of the average atomic mass unit (amu)
RECALL: Atomic Mass (Average atomic mass, atomic weight)
1. Atomic masses are the average of the naturally occurring isotopes of an element
2. Atomic mass does not represent the mass of any actual atom (Think about it…..does it really
make sense that 1 atom of carbon weighs 12.01 grams!!!!)
3. Instead scientist “made up” a unit for the periodic table and called it amu, It gets a little
complicated the way scientists came up with those specific numbers on the periodic table, but
for simplicity sake lets just agree that the number are based on the carbon atom. Why the
carbon atom you ask? Because carbon is found in EVERY living thing on the earth. So the
mass of carbon is a basis for all the other masses on the periodic table. Again for simplicity
sake think of the masses on the periodic table as a ratio compared to carbon.
Back to the mole……..
The mole is abbreviated mol is the number equal to the number of carbon atoms in 12.01g of
carbon.
1. Avogadro's number (in honor of Avogadro-he did NOT discover it) = 6.022 x 1023 units = 1
mole
one mole of something consists of 6.022 x 1023 units of that substance
a unit can be atoms, molecules, particles
in other words:
1mole of C = 12.01 grams of C = 6.022 x 1023 atoms of carbon
1 mole of H = 1.008 grams of H = 6.022 x 1023 atoms of H
***The term “mole” just refers to a particular number. Just as “pair” means “two” and dozen
means “12”, the word “mole” means 6.02 x 1023. In other words 1 dozen eggs = 12 eggs. I
mol of eggs = 6.022 x 1023eggs
Q: If moles are so important, why haven’t I heard of them before?
A: You haven’t heard of it before because it doesn’t make sense to use moles in your everyday life. “Pairs” you
use, because you frequently need two of something. “Dozens” you know because it’s occasionally handy to
have 12 things. However, there are no items in your everyday life that you will ever need 6.02 x 1023 of. After
all, a mole of pencils would weigh six thousand million million million pounds. Unless you do a lot of writing,
this probably won’t come in handy. On the other hand, if you’ve got 6.02 x 1023 of something really tiny (such
as atoms or molecules), moles start being useful. Now, don’t get the wrong idea: We’re not going to use the
number of molecules in any calculations – that would be stupid and pointless, given how small a molecule is.
Instead, we’re going to use something that works the same way as a molecule in the equations. How does this
work? I’m glad you asked!
Let’s say that we want to make a really big bouquet for a wedding.2 If the bride specifies that she wants 36
daisies, 12 roses, and 24 of some other flower3, we can either go to the florist and ask for the numbers she
specified, or we could go to the florist and ask for 3 dozen daisies, a dozen roses, and 2 dozen of whatever that
other flower is. Converting the raw numbers to “dozens” makes our life a little easier in this case.
In the same way, we use moles to make our lives easier when doing chemical reactions. Instead of saying that
we want 1.20 x 1024 molecules of hydrogen and 6.02 x 1023 molecules of oxygen to make water, we can just say
that we want 2 moles of hydrogen and 1 mole of oxygen. Converting to moles doesn’t change the reaction, but
it does make it a lot less annoying. In other words, those coefficients in front of the compound formulas in
chemical equations don’t just refer to molecules, they also refer to moles.
Chemistry Fun Fact
So, why is the number of things in a mole equal to 6.02 x 1023? It’s so that the number of grams in a mole of an
element will be equal to the mass of one atom of the same element in atomic mass units (amu).
8.4 Molar Mass
A. Molar Mass – mass (in grams) of 1 mole of a substance. Also sometimes called formula
weight or molecular weight
Why Are We Doing This, Anyway?
The reason we need to convert between grams and moles is that the only convenient way of measuring
compounds is by mass. This is for two reasons: 1) It’s very hard to count individual molecules/atoms of a
compound; 2) You’d have to do an awful lot of counting to make any meaningful quantity of chemical. Rather
than trying to count a whole bunch of water molecules, it makes more sense to just calculate moles from grams
directly.
RULE – round all molar masses to a whole number EXCEPT chlorine. Cl = 35.5g/mol
Ex. How many moles are in 56g of aluminum?
1 mol of Al = 27g of Al = 6.022 x 1023 atoms of Al
B. The sum of the masses of the component atoms in a compound
a. Molar mass of ethane (C2H6):
Mass of 2 moles of C = 2(12 g) = 24
Mass of 6 moles of H = 6(1 g) = 6.
30 g (with sig fig rules)
**1 mol of C2H6 = 30g of C2H6 = 6.022 x 1023 molecules of C2H6
Ex. Calculate the mass of 4.56 mol of C6H12O6
Ex. How many atoms are in 14.34 g of O?
1 mol of O = __________ g of O = 6.022 x 1023 _________ of O
Ex. Calculate the mass of 1.48 mol of potassium oxide
1 mol of ______ = ________g of _________ = 6.022 x 1023 ___________ of _________
8.5 Percent Composition of Compounds – the percentage by mass of each element in a
compound is known as the percentage composition of the compound.
Mass % =
mass of each element in 1 mol of a compound
Mass of 1 mol of a compound
x 100%
Ex. Determine the mass percent of each element in isopropyl alcohol (C3H7OH)
First find the molar mass of isopropyl alcohol
C- 12g x 3 = 36g
HOTotal mass
Mass % of C=
Mass% of H=
Mass % of O=
Let’s say that you’re really bored and want to find out how much calcium is in your calcium supplement. To do
this, you’d need to find the percent composition of calcium that’s present.
Fun fact: Pure calcium is a poor choice for a dietary supplement, given its strong reactivity with water.
lets say that your calcium supplement contains calcium carbonate.
How much calcium is in 750.0 mg of this supplement?
To figure this out, we first need to figure out how much of the supplement is actually calcium. This means that
we’ll need to find out what percent of the supplement is calcium.
Once we have a percent composition, it’s easy to figure out how much calcium is actually in the supplement. If
40.04% of the supplement is actually calcium, then the total mass of calcium in the supplement is 40.04% of
750.0 mg, or 300.3 mg. NOW YOU TRY.
Ex Determine the mass of hydrogen in 8.00g of H2O
HOTotal mass
Mass % of H
Mass % of O
If H is ______% of the total mass of the molecule H2O, then what mass of the 8g of H2O is
hydrogen?
______ = _______
100%
8.00g
8.6 Determining the Empirical Formula of a Compound
Empirical formula – is the simplest ratio of atoms present in a molecule
Rhyme: percent to mass
Mass to mole
Divide by small
Multiply ‘til whole
Ex A compound consists of 72.2% magnesium and 27.8% nitrogen by mass. What is the
empirical formula?
1. percent to mass: since no mass is given, assume 100g of the substance, which gives us:
2. mass to moles:
Mg-
N-
3. Divide by small
Mg -
N-
4. multiply ‘til whole
Mg-
N-
8.8 Calculation of Molecular Formula
In order to calculate the molecular formula, you must be given both the percent composition
and the molar mass
Ex. A white powder is analyzed and found to have an empirical formula of P2O5. The
compound has a molar mass of 283.88g. What is the compound’s molecular formula?
First determine the empirical formula mass.
PO-
Next, divide the molar mass by the empirical mass formula. This should give you a whole
number.
Ex. Caffeine is a compound containing carbon, hydrogen, nitrogen, and oxygen. The mass
percent composition of caffeine is 49.7% carbon, 5.191% hydrogen, 28.6% nitrogen and
16.48% oxygen. The molar mass of caffeine is approximately 194g/mol. Determine the
molecular formula for caffeine.
Remember if no mass is given, then assume 100g of the substance