Question

Calculate the molecular masses of the following compounds
${H_2},{O_2},C{l_2},C{O_2},C{H_4},{C_2}{H_6},{C_2}{H_4},N{H_3},C{H_3}OH$

Answer 1

Hint: Before calculating the molecular mass we must know the atomic mass of the atoms in the molecule. Therefore atomic mass is the mass of an atom which is accurately determined from the C-12 as the standard system. One atomic mass unit is defined as a mass exactly equal to the 1/12th the mass of one C-12 atom.

Complete step by step answer:
The masses of all other atoms are relative to this standard system described above. Now let us understand what we mean by molecular mass: It is the sum of atomic masses of the elements present in a molecule. It can therefore be obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together. let us analyze the given compounds one by one and find out their molecular mass by multiplying its atomic mass to the number of atoms present and adding them. The first molecule is ${H_2}$ (dihydrogen), we know that atomic mass of hydrogen is 1 u
Molecular mass=2 $\times$ atomic mass of H
=2 $\times$1=2 u
The second molecule is of oxygen, where oxygen has atomic mass of 16 u. therefore,
Molecular mass=2 $\times$ atomic mass of O
=2 $\times$16=32 u
The third molecule is $C{l_2}$ where we know that atomic mass of Cl is 35.5 u. therefore we get
Molecular mass=2 $\times$ atomic mass of Cl
=2 $\times$ 35.5=71 u
The above three molecules are homoatomic molecules that means the molecules are made of only one type of atom. Though all the other molecules are heteroatomic. Molecular mass of carbon dioxide is,
Molecular mass=atomic mass of C + 2 $\times$ atomic mass of O
=12+2(16)=12+32=44 u
Similarly the molecular mass of methane is atomic mass of C+4 $\times$ atomic mass of hydrogen=12+4(1)=16 u,
Molecular mass of ethane is 2 $\times$ atomic mass of carbon+6 $\times$ atomic mass of hydrogen=2(12)+6=30 u,
Molecular mass of ethene is 2 $\times$ atomic mass of carbon+4 $\times$ atomic mass of hydrogen=2(12)+4=28 u,
Molecular mass of ammonia is atomic mass of N+3 $\times$ atomic mass of hydrogen=14+3=17 u and
Molecular mass of methanol is atomic mass of C+4 $\times$ atomic mass of hydrogen + atomic mass of
Oxygen=12+4+16=32 u.
Therefore these are the molecular masses of the above molecules.

Note: Amu “atomic mass unit” is replaced by ‘u’ which is known as unified mass. ‘u’ is used as a standard notation nowadays for denoting mass units.