6.4 – Molar Mass
Molar Mass – _____________________________________________________________________________________________
 Units are grams/mole (g/mol)
 Uses the symbol M (mass is m)
To calculate molar mass, add up the atomic mass of all atoms of the element/compound.
Examples: Calculate the molar mass of:
 Neon (Ne)
 Oxygen (O2)
 Sodium chloride (NaCl)
 Phosphoric acid (H3PO4)
 Ammonium carbonate (NH4)2CO3
Calculations Involving Molar Mass
Equation:
moles = mass/Molar Mass
or
n = m/M
1. Mass (m) to moles (n)
How many moles are in 4.00 g of sodium hydroxide?
2. Moles (n) to mass (m)
What is the mass of 3.00 mol of aluminum chloride?
6.5 – Mass and Number of Entities
Equation:
# of entities = (moles) x (Avogadro’s number)
or
N = nNA
Summary of Symbols and Units
Symbol
n
Quantity
Unit
Amount (in moles)
m
M
g
Molar mass
N
N
Atoms, ions, molecules
A
Avogadro’s constant: 6.02 x 10
23
----
Mass and Number of Entities
1. Determining the Number of Entities in a Sample
Example 1: Calculate the number of atoms in 2 mols of potassium
Example 2: Calculate the number of mols in 4.22 x 1021 atoms of sulfur.
2. Determining the Number of Entities in Compounds
Example 3: Calculate the number of mols in 5.5 x 1022 molecules of H2O
Example 4: a) Calculate the number of molecules in 5 moles of water.
b) How many HYDROGEN ATOMS are in this sample?
Combining the Formulas
Example 5: Calculate the number of atoms in a 1000 g bar of gold
Example 6: Calculate the mass of 4.35 x 1024 molecules of ammonia (NH3)
Example 7: Calculate the number of each atom in a 26.5 g sample of benzaldehyde (C 6H5CHO), a
compound used for almond flavouring.