Mole Calculation

The equation that links the mass of a substance to the number of moles present is:

mass = number of moles × molar mass

where mass is in grams and the molar mass is in grams per mole.

Moles to Mass Calculation

We can use the above equation to find the mass of a substance when we are given the number of moles of the substance.

Example:

Calculate the mass of (a) 2 moles and (b) 0.25 moles of iron. (Relative atomic mass: Fe = 56)

Solution:

a) mass of 2 moles of iron

= number of moles × molar mass

= 2 × 56

= 112 g

b) mass of 0.25 mole of iron

= number of moles × molar mass

= 0.25 × 56

= 14 g

Example:

Calculate the mass of (a) 3 moles and (b) 0.2 moles of carbon dioxide gas, CO2. (Relative atomic mass: C = 12; O = 16)

Solution:

a) mass of 1 mole of CO₂

= (1 × 12) + (2 × 16)

= 44 g

mass of 3 moles of CO₂

= 3 × 44

= 132g

b) mass of 0.2 mole of CO₂

= 0.2 × 44

= 8.8 g

The following videos show some examples of moles to mass calculation.

Mass to Moles Calculation

If we are given the mass of a substance and we are asked to find the number of moles of the substance, we can rewrite the above equation as

Example:

Calculate the number of moles of aluminum present in (a) 108 g and (b) 13.5 g of the element. (Relative atomic mass: Al = 27)

Solution:

a)

b)

Example:

Calculate the number of moles of magnesium oxide, MgO in (a) 80 g and (b) 10 g of the compound. (Relative atomic mass: O = 16, Mg = 24)

Solution:

a) Mass of 1 mole of MgO

= (1 x 24) + (1 x 16)

= 40 g

b)

The following video shows an example of mass to mole calculation.

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