Quantities in Chemistry
The Mole and Molar Mass
Mole Review
• A Mole is a unit of measurement in chemistry. It
represents 6.02 x 1023 of an entity. One mole of
sodium is 6.02 x 1023 atoms of sodium. The
symbol of mole is “mol”.
• 6.02 x 1023 is also known as Avogadro’s constant.
Molar Mass
• Molar Mass (M) is the mass (in grams) of one
mole of a chemical entity.
• The formula for molar mass is:
• Molar Mass (M) = mass (g) / mole (mol)
• The molar mass of an element can be determined
from the periodic table. It is the same number as
the atomic mass.
• For example Hydrogen’s molar mass is 1.01 g/mol
Molar Mass of Compounds
• The molar mass of a compound is equal to
the sum of the molar masses of the atoms in
the molecules or ions in the compound.
• For example, the molar mass of water is:
• MH2O = 2(MH) + (MO)
= 2(1.01g/mol) + 1(16.0g/mol)
=18.02 g/mol
Example: Find the molar mass of
H2CO2
• M H2CO2 = 2(MH) + (MC) +2(MO)
= 2(1.01g/mol) + (12.00g/mol) + 2(16.0g/mol)
=46.02 g/mol
Calculations Involving Atoms,
Moles, Mass & Molar Mass
• It is often more convenient and practical to
determine the number of atoms using mass
instead of counting.
• It is easier to determine mass by using
number of moles
Atoms, Moles, Mass & Molar
Mass Calculations
• Example 1: Calculate the molar mass of
magnesium if 0.330mol has a mass of 8.02g
M=m/n
= 8.02g/0.330mol
= 24.3g/mol
Atoms, Moles, Mass & Molar
Mass Calculations
• Example 2: Calculate the mass, in grams, of
2.00 mol of calcium atoms.
M=m/n
m=Mxn
= 40.08g/mol x 2.0 mol
= 80.2g
Atoms, Moles, Mass & Molar
Mass Calculations
• Example 3: How many moles of gold are in
275.8g of pure gold?
M=m/n
n = m/M
= 275.8g / 197.0 g/mol
= 1.400 mol
Calculating Number of Atoms
• If you have a mass of the number of moles
of an item, you can determine the number of
molecules in an amount of an element using
Avogadro’s number.
• Avogadro’s number = 6.02 x 1023 atoms/mol
Atom Calculations
• Example 4: How many atoms of gold are in a
232.8g of pure gold?
• Step 1: Determine the number of moles
– n = m/M
= 232.8g / 197.0g/mol
= 1.182 mol
• Step 2: Using Avogadro’s number and the number
of moles, determine the number of atoms.
– N= n x NA
= 1.182 mol x 6.02 x 1023 atoms/mol
= 7.12 x 1023 atoms
Atom Calculations
• Example 5: How many atoms of sulfur are in a
230.0g of pure sulfur?
• Step 1: Determine the number of moles
– n = m/M
= 230.0g / 32.06g/mol
= 7.174 mol
• Step 2: Using Avogadro’s number and the number
of moles, determine the number of atoms.
– N= n x NA
= 7.174 mol x 6.02 x 1023 atoms/mol
= 4.319 x 1024 atoms