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Moles, Mass, & Molar Mass

The Mole

The mole is a term used to describe a fixed number of particles.
In everyday life we use the term 'dozen' to describe 12 items.
Just as a 'dozen eggs' describes 12 eggs, a mole always represents 6.02 x 10²³ particles. This number is known as Avogadro's number.
This is an enormous number of particles, and it is easier to deal with the 'mole'.
A mole can be defined as the amount of any substance that contains as many particles as there are in 12 g of carbon-12.
The terms 'amount of substance' and 'number or moles' means the same thing.
The symbol for the mole is 'n', and the unit is 'mol'.

The mass of one mole of an element, compound or ion is the molar mass, M.
Molar mass has the unit g mol^-1
The molar mass of any substance is the sum of the molar masses of the atoms, or ions in the formula.
Molar mass values of elements are found on the periodic table

For the following examples the molar mass values (in g mol^-1 ) from the periodic table are:

H = 1.0, C=12.0, S = 32.1, O = 16.0, Cu = 63.6

EXAMPLE 1: The molar mass of H₂

= 2 x 1.0 = 2.0 g mol^-1

EXAMPLE 2: The molar mass of O₂

= 2 x 16.0 = 32.0 g mol^-1

EXAMPLE 3: The molar mass of CuSO₄ (copper sulfate)

= 63.6 + 32.1 + (4 x 16.0) = 159.7 g mol^-1

EXAMPLE 4: The molar mass of C₆H₁₂O₆ (glucose)

= (6 x 12.0) + (12 x 1.0) + (6 x 16.0) = 180.0 g mol^-1

EXAMPLE 5: The molar mass of H₂SO₄ (sulfuric acid)

= (2 x 1.0) + 32.1 + (4 x 16.0) = 98.1 g mol^-1

The equation n = m/M describes the relationship between n (the amount in mol), m (the mass in g), and M (the molar mass in g mol^-1 )

This equation can be re-arranged. The triangle below will help you to re-arrange the equation.

m = nM

e.g. The mass of 2 mol of magnesium atoms, M (Mg) = 24.3 g mol^-1

m = n x M = 2 x 24.3 = 48.6 g

n = m/M

e.g. The amount of calcium oxide in 154 g, M (CaO) = 56.1 g mol^-1

n = m/M = 154/56.1 = 2.75 mol

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