C4 – Chemical Calculations:
GCSE Chemistry Guide
1. Relative Formula Mass (Mr)
The relative formula mass (Mr) is the sum of the relative atomic masses of all atoms in a
compound.
Example: Mr of H₂O = (2 × 1) + 16 = 18
2. The Mole
A mole is the amount of a substance that contains 6.022 × 10²³ particles (Avogadro's
number).
Number of moles is calculated using the formula:
mass (g)
Moles=
Mr
3. Mass Calculations
To calculate masses in reactions:
- Write the balanced equation.
- Calculate moles of known substance.
- Use the mole ratio to find moles of unknown (using the numbers in the balanced equation)
- Convert moles to mass.
Example:
What mass of magnesium oxide (MgO) is formed when 12 g of magnesium (Mg) is burned in
oxygen?
Step 1: Write the Balanced Equation
2Mg + O₂ → 2MgO
Step 2: Calculate Moles of Known Substance (Magnesium)
Mr of Mg = 24moles of Mg = mass ÷ Mr = 12 ÷ 24 = 0.5 mol
GCSE Chemistry Guide
1. Relative Formula Mass (Mr)
The relative formula mass (Mr) is the sum of the relative atomic masses of all atoms in a
compound.
Example: Mr of H₂O = (2 × 1) + 16 = 18
2. The Mole
A mole is the amount of a substance that contains 6.022 × 10²³ particles (Avogadro's
number).
Number of moles is calculated using the formula:
mass (g)
Moles=
Mr
3. Mass Calculations
To calculate masses in reactions:
- Write the balanced equation.
- Calculate moles of known substance.
- Use the mole ratio to find moles of unknown (using the numbers in the balanced equation)
- Convert moles to mass.
Example:
What mass of magnesium oxide (MgO) is formed when 12 g of magnesium (Mg) is burned in
oxygen?
Step 1: Write the Balanced Equation
2Mg + O₂ → 2MgO
Step 2: Calculate Moles of Known Substance (Magnesium)
Mr of Mg = 24moles of Mg = mass ÷ Mr = 12 ÷ 24 = 0.5 mol
