AQA GCSE FOUNDATION Chemistry: Calculating Relative Formula Mass (Mr)

Calculating Relative Formula Mass (Mr)

The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in its formula.

Here's how to calculate Mr:

1. Identify the chemical formula of the compound. For example, the formula for water is H2O.

2. Look up the relative atomic masses (Ar) of each element in the periodic table.

   • Ar(H) = 1
   • Ar(O) = 16

3. Multiply the Ar of each element by the number of atoms of that element in the formula.

   • For H2O:
     • 2 x Ar(H) = 2 x 1 = 2
     • 1 x Ar(O) = 1 x 16 = 16

4. Add the results from step 3 to find the Mr.

   • For H2O: Mr = 2 + 16 = 18

Example:

Calculate the Mr of sodium chloride (NaCl):

1. Formula: NaCl
2. Ar(Na) = 23, Ar(Cl) = 35.5
3. 1 x Ar(Na) = 1 x 23 = 23 1 x Ar(Cl) = 1 x 35.5 = 35.5
4. Mr(NaCl) = 23 + 35.5 = 58.5

Remember:

• The unit for Mr is the same as for Ar, which is amu (atomic mass unit).
• You can use the periodic table to find the Ar values.
• Always pay attention to the subscripts in the chemical formula to indicate the number of atoms of each element.

Practice makes perfect! Try calculating the Mr of different compounds to get comfortable with this process.