Question

The mass composition of the universe may be given as 90% H2 and 10% He. The average molecular mass of the universe should be
A.2.20
B.2.10
C.3.80
D.3.64

Answer 1

Molecular mass is defined as the mass of a given molecule and it is measured in daltons (Da or u) and average molecular mass is the mass of the compound divided by the number of molecules.

Complete step by step answer: Here to calculate the average molecular mass, we will start with the given composition. So as per given our compound will have 90% of Hydrogen (${H_2}$) and 10% of $He$. For Hydrogen, the molecular mass is 1 but since it exists as ${H_2}$ hence, its molecular mass will be 2 and for the case of Helium ($He$) the molecular mass is 4. So the mass of the compound is calculated as:
$0.9 \times 2 + 0.1 \times 4 = 2.2g$
The total number of molecules is calculated as the sum of the total number of molecules of Hydrogen and the total number of molecules of Helium which is equal to $0.9 + 0.1 = 1$
Hence the average molecular mass of the universe is calculated as $\dfrac{{2.2}}{1} = 2.20$
Hence the correct answer is option A.

Note: Finding molecular mass is relatively easy for small molecules but it becomes difficult for polymers and proteins since these atoms are very large. For the case of larger molecules, the molecular weight is found using different approaches such as viscosity and light scattering. The larger molecules discussed here are DNA, proteins, etc.