Calculate the molecular masses of hydrogen bromide ( $ HBr $ ).
(Atomic masses: $ H = 1u;Br = 80u $ )

Hint: The molecular or molar mass of any chemical compound refers to the ratio of mass of a sample of that particular compound and the amount of substance in that particular sample (in moles). The molar mass is actually a bulk characteristic of a substance rather than molecular. The molar mass is generally represented as $ gmo{l^{ - 1}} $ .

Complete step by step solution:
 The molecular mass of any compound can be found out by adding the relative atomic masses of each element present in that particular compound. The number of atoms in a compound can be determined from their chemical formula. In the question we are given the following atomic masses:
 $ \begin{gathered}
  H = 1u \\
  Br = 80u \\
\end{gathered} $
Now, let us calculate the molecular mass of the given compound i.e. hydrogen bromide. The chemical formula of hydrogen bromide is given i.e. $ HBr $ .
From the chemical formula, it is clear that Hydrogen bromide contains one atom of hydrogen and one atom of bromine and thus molecular mass of this compound can be calculated by adding the mass of one hydrogen atom and one bromine atom as shown below:
 $ Molecular{\text{ }}mass{\text{ }}of{\text{ }}HBr = (1 \times H) + (1 \times Br) = (1 \times 1) + (1 \times 80) = 81u $

Note:
Molecular mass plays a significant role in chemistry especially during setting up an experiment. During testing principles which involve specific amounts or quantities of a substance, molecular mass is used to figure out the exact quantity to be weighed of that particular substance. Basically molecular mass is used to determine the stoichiometry in the chemical reactions as well as equations.