The molar mass is calculated by adding up the relative atomic masses of all the different species and multiplying them by the quantity of each. Let's start by recording the relative atomic masses of each species.
One barium atoms = 1 * 137.327 = 137.327
Two Bromine atoms = 2 * 79.904 = 159.808
Total = 297.135
Molar mass = 297.135 g/mol (take note that the molar mass unlike the relative atomic mass has units)
