Question

The relative molecular mass of methane, $ C{H_4} $ is $ 16.043 $. What is the relative molecular mass of ethane $ {C_2}{H_6} $ and benzene $ {C_6}{H_6} $ ?

Answer 1

Hint: The relative molecular mass of any compound is determined by taking ratio of average mass of one molecule of particular element to $ {\dfrac{1}{{12}}^{th}} $ mass of an atom of carbon $ \left( {{C^{12}}} \right) $ . Molecular energy is expressed in unit Daltons.

Complete answer:
The relative molecular mass of a compound is calculated by adding atomic mass of its constituent elements. Atomic mass is an atom is determined by taking ratio of mass of an particular atom to the $ {\dfrac{1}{{12}}^{th}} $ mass of a carbon atom $ \left( {{C^{12}}} \right) $ .
As we know atomic mass is expressed as a.m.u, molecular mass is also expressed in a.m.u representing the molecular mass in terms of a.m.u only shows that mass of each atom is considered in atomic mass scale unit.
Molecular mass of $ C{H_4} $ is $ 16.043 $ . Atomic mass of hydrogen atom $ H $ is $ 1 $ and atomic mass of carbon atom $ C $ is $ 12 $ .
Molecular mass of $ {C_2}{H_6} $ $ = $ $ 2 \times $ mass of carbon atom $ + $ $ 6 \times $ mass of hydrogen atom
 $ {C_2}{H_6} $ $ = 2 \times 12 + 6 \times 1 $
After solving the above equation, we get
 $ {C_2}{H_6} $ $ = 24 + 6 $
 $ {C_2}{H_6} $ $ = 30 $
Molecular mass of $ {C_6}{H_6} $ $ = $ $ 6 \times $ mass of carbon atom $ + $ $ 6 \times $ mass of hydrogen atom
 $ {C_6}{H_6} $ $ = 6 \times 12 + 6 \times 1 $
After solving the above equation, we get
 $ {C_6}{H_6} $ $ = 72 + 6 $
 $ {C_6}{H_6} $ $ = 78 $
 $ \Rightarrow $ The relative molecular mass of methane $ C{H_4} $ is $ 16.043 $ .
 $ \Rightarrow $ The relative molecular mass of ethane $ {C_2}{H_6} $ is $ 30 $ .
 $ \Rightarrow $ The relative molecular mass of benzene $ {C_6}{H_6} $ is $ 78 $ .

Note:
There is also the possibility of having different molecular mass of two compounds which are containing similar types of atoms but their isotopes are selected. For example, the mass of $ {C^{12}} $ is $ 12 $ a.m.u and the mass of $ {C^{14}} $ isotope is $ 14 $ a.m.u . Molecular mass and molecular weight are frequently used in other places.