# 3.1.2 Relative Formula Mass

### Relative Formula (Molecular) Mass

• The symbol for the relative atomic mass is Ar
• This is calculated from the mass number and relative abundances of all the isotopes of a particular element
• The symbol for the relative formula mass is Mr and it refers to the total mass of the molecule
• To calculate the Mr of a substance, you have to add up the relative atomic masses of all the atoms present in the formula

Relative Formula Mass Calculations Table • In accordance with the Law of Conservation of Mass, the sum of the relative molecular masses of the reactants will be the same as the sum of the relative molecular masses of the products

#### Exam Tip

The relative atomic mass of every element is given on the Periodic Table. It is the larger of the two numbers.

### Calculating % Mass of an Element in a Compound

• The percentage by mass of an element in a compound can be calculated using the following equation: #### Worked Example

Calculate the percentage by mass of calcium in calcium carbonate, CaCO3 #### Exam Tip

Don’t forget to multiply your answer by 100 in order to convert it to a percentage. ### Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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