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Chemical formulas,
Molar Mass, Molar Volume
&
Mr. Shields
Regents Chemistry
U04 L02
1
Writing Chemical Formulas
 Subscripts indicate the number of atoms of each
element in a representative unit
 If there is no subscript, it is understood to be 1. (Ex.
CaO
– 1 Calcium & 1 Oxygen)
 Subscripts refer to the element immediately
preceding them (Ex. H2S – 2 Hydrogen).
 Subscripts after parentheses refer to everything
inside (Ex. Ca(OH)2 – 2 Oxygen & 2 Hydrogen)
2
Chemical Formulas
 What atoms and how many of each are present
in the following compounds?
 CH4 ?
 Ba(CN)2 ?
 Cr2O3 ?
 Cr(OH)3 ?
3
Number of atoms in formulas
Subscript outside ( ) x subscript
inside ( ) = # of atoms
 How man atoms of each element are in
Cr2(NO3)3
 Cr = 2
N = 1 X 3 = 3
O = 3 X 3 = 9
 Total = 14
4
Coefficients
 Numbers in front of formulas
 Multiply the total # of atoms for each
element in the formula by the coefficient
 Example: 2H2O


2 X 2 = 4 H’s
2 X 1 = 2 O’s
5
Hydrates
 A group of salts that have water molecules
stuffed in the empty spaces
 Formulas are distinctive
 Ex: CuSO45H2O


“” means “is associated with”
Not a true chemical bond between the water and the
compound, but the structure is always the same and
the number of water molecules is always the same
6
5CuSO45H2O
 How many atoms are there?

(note the coefficient & the water of hydration)
Coefficients x Subscripts
5 X Cu
5XS
20 X O
5x5x2 H
5x5 O
Total # atoms = 5+5+20+50+25 = 105
7
CuSO45H2O
 What is the % water in CuSO4  5H20?
First calculate the formula mass:
1 X Cu =
1XS =
4XO =
5x2 H =
5x1 O =
63.546
32.066
15.999 x 4
1.008 x 10
15.999 x 5
Second find the % water
(90.075 / 249.687) x 100 =
36.08% water
formula mass = 249.687amu
8
Ba(OH)28H2O
What is the % water in this compound?
 Count the atoms first
1 x Ba
2x O
2XH
16 X H
8XO
Molar Mass = 315.47 g/mol
Then multiply # of atoms by
gram atomic mass
137.33 g
32.
2.O1
16.13
128.
Ba
O
H
H
O
% H20 = (144.13/315.47) x 100
= 45.69%
9
10
A Real Mole
No, we are not
Talking about
This kind of mole!
11
The Mole (Mol) is the SI unit for Counting
Just like 1 Dozen = 12 of anything
1 Mole = 6.022 X 1023 of anything
How much is a mole of pennies?
A mole of pennies would
Cover the entire Earth
to a depth of 300 meters
12
Remember… 1 mole = 6.02 x 1023 atoms =
an elements Gram atomic mass
13
The gram molecular mass is obtained by
Multiplying the number of atoms of each element
By it’s gram atomic mass. Let’s consider hexane…
For example the Gram molecular mass of Hexane (C6H14) is
6C + 14H = 6(12.011) + 14(1.008) = 86.178g
So …
1 mole = 86.178g =
3 moles = 258.534g =
And 1/3 mole =
?
6.022 x1023 molecules
1.807 x 1024 molecules
14
MOLAR MASS
Definition: Molar mass is the mass in grams of one mole
(or 6.02 x 1023 molecules) of any element or chemical
compound
Therefore, the molar mass = gram molecular mass or gram
formula mass or gram atomic mass of the element or compound
What is the molar mass of Sr(C2H3O2)2 ?
87.62 + (4*12.01) + (6*1.01) + (4*16) = 205.72 g/mol
15
MOLAR MASS
Problem: If the molar mass of Sr(C2H3O2)2 is 205.72g/mol
then how many moles are in 134.75g?
134.75g / 205.72g/mol = 0.66 moles
16
17
Molar Volume
# N2
100L
=
# CO2
100L
Back to Avogadro …
Avogadro proposed that equal volumes of gases at the
same temp and pressure contained equal numbers of
particles. This was known as AVOGADRO’S HYPOTHESIS
For example:
100 liters of N2 at the same temperature and pressure
has the same # of molecules as 100 liters of CO2
18
Molar Volume
22.4L
So 1 mole of any gas occupies the same volume as 1 mole of
any other gas at the same temp and pressure
Which means 6.023 x 1023 molecules of any gas occupies the
same volume for any gas
The volume occupied by one Mole of any gas = 22.4 Liters
This is called the
MOLAR VOLUME (Abbreviated “ Vm “)
19
Molar Volume
If equal volumes of a gas have
An equal no. of molecules
Molar mass of CO2 = 44g
And …
22.4L equals one mole of any gas
Then…
22.4L contain a molar mass of any gas
AT STP !
20
Molar Volume Problems
PROBLEM 1: If one mole of a gas at STP occupies 22.4L,
how many liters will two moles occupy?
22.4 L x 2 Moles = no. of liters = 44.8 Liters
1 mole
Problem 2: How many moles are there in 14L of O2 at STP?
14 Liters /
22.4L
1 mole
= # mole = 0.625mol
PROBLEM 3: What is the mass of 1L of O2?
(1L / 22.4L) x 32 = 1.43g
21