Relative Atomic Mass

Atoms are so tiny that we cannot really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass (A_r) is used
The relative atomic mass unit is equal to 1/12th the mass of a carbon-12 atom
All other elements are measured by comparison to the mass of a carbon-12 atom and since these are ratios, the relative atomic mass has no units
For example, hydrogen has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon

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

$A_{r} = \frac{(% of isotope A x mass of isotope A) + (% of isotope B x mass of isotope B)}{100}$

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

The table shows information about the isotopes in a sample of rubidium

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

Give your answer to one decimal place.

Answer:

You will be provided with the equation to calculate relative atomic mass in an exam so do not need to be able to recall it.

Relative Atomic Mass (WJEC GCSE Chemistry)