Chapter 6 Chemical Composition
Problem:
You work at a jelly bean store. Someone comes in and asks for 1000
jelly beans.
What would you do so you don’t have to take the time to count exactly
1000 jelly beans?
If the average mass of a jelly bean is 5grams, how would you fulfill the
customer’s order?
In this problem I told you the average mass of a jelly bean. If you had
to figure out the average mass of a jelly bean, how would you do it?
If you weigh 10 jelly beans and find the following results, what would
the average jellybean weigh?
Problem 2:
A new customer comes in and says that she needs jellybeans
for one of her kids and mints for the other kid.
She doesn’t care how much candy she gets, as long as she has
equal numbers of jellybeans and mints.
You happen to know that every jelly bean weighs 5 grams and
every mint weighs 15grams.
You put a scoop of jellybeans on the scale and it weighs
500grams.
How many grams of mints should you get?
There are several strategies.
-use ratios
-use equivalence statements
I. Atomic Masses: Counting by Weighing
Atoms are really small.
The mass of a carbon atom is 1.99 x 10-23 grams.
It doesn’t make sense to weigh atoms in grams
because the number is so small.
Scientist came up with a new unit of mass
for measuring the mass of atoms.
I. Atomic Masses: Counting by Weighing
A. Atomic Mass Unit (amu): A unit of mass that is
used to measure the mass of atoms.
1.
1 amu = 1.66 x 10-24 grams
B. Average Atomic Mass: Tells the average mass of
an atom of an element. (it’s like knowing the
average mass of a jelly bean.)
1.
To find atomic mass, look on the periodic table.
a.
2.
Example: The average atomic mass of carbon = 12.01amu
Average atomic mass can be used as an equivalence
statement. (1 carbon atom = 12.01amu)
a)
b)
What is the mass in amu’s of 198atoms of carbon.?
How many carbon atoms are in 2.50 x 1019 amu’s of
carbon.?
c)
d)
e)
f)
g)
What is the mass of 75 atoms of aluminum?
How many nitrogen atoms are in 1387amu’s of nitrogen?
What is the mass of 1 molecule of H2O?
How many molecules of water are in 630.70amu’s of water.
What is the mass in amu’s of 45molecules of CH4
Back to Jellybeans and Mints
-Remember that a jellybean in 5grams and a mint is 15grams.
1. For each of the following, fill in the blank so that the number
of jellybeans and mints will be the same.
a.
15
5g Jellybeans and _____g
mints.
b.
15 tons of mints.
5 tons of jellybeans and _____
c.
15 tons of mints.
5lbs of jellybeans and _____
It doesn’t matter what unit you are using, the mass of
mints will always be 3 times larger than the mass of
jellybeans. (or a ratio of 5 to 15)
105
35g of jellybeans and _______g
of mints
35g x 3 = 105
or
15𝑔 𝑀𝑖𝑛𝑡𝑠
35g jellybeans x 5 𝑔 𝑗𝑒𝑙𝑙𝑦𝑏𝑒𝑎𝑛𝑠 = 105g mints
30 tons of mints and ______tons
of jellybeans.
10
30tons ÷ 3 = 10tons
or
30 tons mints x
5 𝑡𝑜𝑛𝑠 𝑗𝑒𝑙𝑙𝑦𝑏𝑒𝑎𝑛𝑠
15 𝑡𝑜𝑛𝑠 𝑀𝑖𝑛𝑡𝑠
= 10 tons jellybeans
Lets apply this to atoms
1. For each of the following, fill in the blank so that the number
of aluminum atoms and copper atoms will be the same.
a.
26.98amu’s Al and _____amu’s
Cu.
63.55
b.
26.98g Al and 63.55
_____g Cu.
2. How many atoms were we referring to in letter a? 1 of each
3. How many atoms were we referring to in letter b? ???????????
We know that when we look at the periodic table
12.01g Carbon
16.00g Oxygen
63.55g Copper
14.01g Nitrogen
26.98g Aluminum
They all have the same number of atoms.
Atoms are too small to count.
Atoms are also to small for us to weigh in small
quantities.
It would take 500 million trillion carbon atoms to
show up on our scales.
So 500 million trillion is a small quantity of
carbon.
Chemists have come up with a way to weigh
large quantities of atoms.
C.
The MOLE!!!!!!!!: it is defined as the number of atoms in 12.01g
of carbon.
-It could also be defines as the number of atoms in 14.01g of nitrogen
-It could also be defines as the number of atoms in 16.00g of Oxygen
-It could also be defines as the number of atoms in 35.45g of chlorine.
63.55
-It could also be defines as the number of atoms in _________g
of
copper
26.98
It could also be defines as the number of atoms in _______g
of
aluminum
C. The MOLE!!!!!!!!: it is defined as the number
of atoms in 12.01g of carbon.
1. Avagadro’s Number: The actual number of
objects in a mole of something.
a) Avagadro’s Number = 6.022 x 1023
b) 1 mole = 6.022 x 1023 (A very important
equivalence)
This number is so large that
A mole of seconds is 4 million times as long as the earth
has existed (according to science).
A mole of marbles would cover the earth 50 miles deep.
That’s how large a mole is.
However, a mole of carbon atoms fits in a beaker.
That’s how small atoms are.
2. Think of a mole as being like a dozen, but a whole lot
larger.
•
•
1 dozen = 12 of something
1 mole = 6.022x 1023 of something
3. Determining numbers of atoms (use avagadro’s number)
a.
5.78moles of carbon = _______atoms?
b.
5.78moles of oxygen = _______atoms?
c.
5.78 moles of marbles = _______marbles?
d.
9.38 x 1023 copper atoms = _________moles?
e.
4.98 x 1024 molecules of water = _________ moles?
D. Molar Mass(M.M.): The mass of one mole
of a substance.
1. The same # as the average atomic mass
a.
b.
Example: 1 mole C = 12.01g
Example: 1 mole H2O = 18.02g
c. What is the molar mass of NH3 ?
d. What is the mass of one mole of CH4 ?
e. What is the mass of 6.022 x 1023 atoms of chlorine?
2. Molar Mass as an equivalence statement.
M.M. = 1mole = 6.022 x 1023 units
The M.M. of carbon is 12.01g/mol.
So
12.01gC = 1mole
And
12.01g C = 6.022 x 1023 atoms
12.01gC = 1mole
And
12.01g C = 6.022 x 1023 atoms
How many atoms are in 35.6g of carbon?
35.6g C x
6.022𝑥10^23 𝑎𝑡𝑜𝑚𝑠 𝐶
12.01𝑔 𝐶
= 2.14 x 1025 atoms C
How much does 5.98moles of carbon weigh?
12.01𝑔 𝐶
5.98 moles C x 1 𝑚𝑜𝑙𝑒 𝐶 = 71.82g C
How many moles of water are in 27.6g of H2O?
1𝑚𝑜𝑙 𝐻2𝑂
27.6g H2O x 18.02𝑔 𝐻2𝑂 = 1.53mol H2O
II. % Composition
A. Mass Percent: Tells the percentage of each
element in a compound. (based on mass)
Mass % =
𝑇𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑎𝑛 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑖𝑛 1 𝑚𝑜𝑙𝑒 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑
𝑚𝑎𝑠𝑠 𝑜𝑓 1 𝑚𝑜𝑙𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑
x
100%
What are the mass percent's of each element in ethanol
(C2H6O)?
24.02𝑔
Mass 2 mol C = 12.01g x 2 = 24.02g
% C= 46.08𝑔 x 100% =52.1%
Mass 6 mole H = 1.01g x 6 = 6.06g
% H= 46.08𝑔 x 100% =13.2%
Find the mass % of each element in water.
6.06𝑔
16.00𝑔
Mass 1 mole O = 16.00 x 1 = 16.00g % O=
x 100% = 34.7%
46.08𝑔
Mass 1 mole Compound = 46.08g
III. Formulas of Compounds
A. Molecular Formula: The actual ratio of atoms in
a compound.
1.
Example: C6H12O6
shows a ratio of 6 carbon to 12 hydrogen to 6 oxygen
B. Empirical Formula: The smallest whole number
ratio of atoms in a compound.
1. Examples:
Molecular Formula
C6H12O6
C3H9
C2H5O4
Empirical Formula
2. Calculating an Empirical Formula
a) Steps
1.
2.
3.
4.
Determine how many grams of each element are present.
Calculate how many moles of each element are present.
Calculate the ratio of the moles. (Divide by the smallest)
Write the formula that shows the ration from step 3.
Example: What is the empirical formula of a compound that contains
74.8% C and 25.2% H