Mole, Avogadro Constant & Molar Mass

Mole & Avogadro constant

• An amount of substance containing 6.02 × 10²³ particles is called a mole (often abbreviated to mol).
• 6.02 × 10²³ is called the Avogadro constant.

Example:

One mole of carbon contains 6.02 × 10²³ of carbon atoms

One mole of oxygen contains 6.02 × 10²³ of oxygen molecules

Molar mass

• The mass of one mole of a substance is called the molar mass.

• The molar mass of a substance is equal to its relative formula mass in grams.

Example:

What is the mass of 1 mole of carbon?

Solution:

The mass of 1 mole of carbon = relative formula mass of carbon = 12 grams

Example:

What is the molar mass of calcium carbonate (CaCO₃)?

Solution:

Step 1: Look up the relative atomic masses of the atoms from the periodic table.

Relative atomic mass (rounded to the nearest whole number):

Ca = 40, C = 12, O = 16

Step 2: Calculate the relative formula mass.

Calcium carbonate (CaCO₃) contains one calcium atom, one carbon atom and three oxygen atoms.

Relative formula mass = 40 + 12 + (3 × 16) = 100

Step 3: Express the relative formula mass in grams per mole.

The molar mass of ethanol is 100 g/mol

Example:

What is the molar mass of ethanol (C₂H₅OH)?

Solution:

Step 1: Look up the relative atomic masses of the atoms from the periodic table.

Relative atomic mass (rounded to the nearest whole number):

H = 1, C = 12, O = 16

Step 2: Calculate the relative formula mass.

Ethanol (C₂H₅OH) contains two carbon atoms, six hydrogen atoms and one oxygen atom.

Relative formula mass = (2 × 12) + (6 × 1) + 16 = 46

Step 3: Express the relative formula mass in grams per mole.

The molar mass of ethanol is 46 g/mol

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