Calculate Relative Atomic Mass (Edexcel IGCSE Chemistry)

Spec point — be able to calculate the relative atomic mass of an element (Ar) from isotopic abundances

Calculating relative atomic mass

• The relative atomic mass of each element is calculated from the mass number and relative abundances of all the isotopes of a particular element
• The equation below is used where the top line of the equation can be extended to include the number of different isotopes of a particular element present

A _ r = (( % of isotope A x mass of isotope A ) + ( % of isotope B x mass of isotope B ))/(100)

• So, if there were 3 isotopes present then the equation would read:

$\frac{( \% of \text{isotope} \mathrm{A} \mathrm{x} \text{mass} of \text{isotope} \mathrm{A} ) + ( \% of \text{isotope} \mathrm{B} \mathrm{x} \text{mass} of \text{isotope} \mathrm{B} ) + ( \% of \text{isotope} \mathrm{C} \mathrm{x} \text{mass} of \text{isotope} \mathrm{C} )}{100}$

Worked Example The table shows information about the Isotopes in a sample of rubidium with 72% ⁸⁵Rb and 28% ⁸⁷Rb

Isotope	Mass Number	Percentage abundance
1	85	72
2	87	28

Use information from the table to calculate the relative atomic mass of this sample of Rubidium.

Give your answer to one decimal place.

Answer:

• Relative atomic mass = $\frac{( 72 \times 85 ) + ( 28 \times 87 )}{100}$
• Relative Atomic Mass = 85.6

Exam Hint Isotopes are easy to recognise from their notation as they have the same symbol but different mass numbers.

For example, the two stable isotopes of copper are ⁶³Cu and ⁶⁵Cu