Week 9 Molar Mass Moles and Molecules
Molar Mass, Moles, and Molecules
7.3 Using Chemical Formulas
What is a Mole?
A mole is a number used to describe a quantity of something, usually referring to mass, volume, or the number of particles.
As a unit, it is shortened to mol
Molar Mass
The atomic mass of an element on the periodic table is the mass of 1 mole of that element (also called the
molar mass, measured in g/mol).
In a chemical formula, the subscript represents how many moles of that element are present in that compound.
Multiply the number of moles by the atomic mass to find the mass of an element in a compound.
Add together the masses of all elements in the compound to find the formula mass.
1.
2.
3.
Formula Mass Examples
NaCl =
H
2
O =
CaSO
4
=
Complete the practice problem on page 238.
Converting with Molar Mass
It is possible to find the number of moles of a given formula mass by dividing by the molar mass of each element in a formula.
Ex: How many moles of CO
2
C = , O
2
= are in 88.02 g?
Complete practice problem #1 and 3 on page 240.
Molecules
The quantity of a mole can also be used to find the number of particles of a substance are present (usually measured in molecules or atoms)
1 mole = 6.022 x 10 23 particles
This constant is known as Avogadro’s number.
Converting with Molecules
To find the number of molecules, multiply the number of moles by Avogadro’s number.
Ex: 1 mole of O
2
= ? Molecules
To find the number of moles, divide by Avogadro’s number.
Ex. 1.204x10
24 molecules of O
2
= ? Moles
Complete practice problem #2 on page 242.
Mass, Moles, and Molecules
To convert between all three quantities, you must go through moles. (1 st divide, then multiply)
Ex: 4.04 g of H
2
= ? Molecules of H
2
Complete practice problem #2 on page 242.
Exit Ticket
P. 244 - Complete #1-4 on the section review
