Question

Compute the mass of one molecule and the molecular mass of ${C_6}{H_6}$ (benzene).
(At. Mass of $C = 12$ , $H = 1u$).

Answer 1

We need to know the difference between mass of a molecule and the molecular mass of a compound. In order to calculate the mass of a molecule, we first need to define the molecular mass of that molecule of the given compound. With the given molecular formula of a compound and the atomic masses of its constituent elements, we calculate the molecular mass and then bring in the concept of Avogadro’s number to calculate the mass of one molecule of a compound.

Complete step by step answer:
The molecular mass of a compound is calculated using the following rules:
The atomic masses of each of the elements constituting the given compound must be known.
We multiply the atomic mass of a particular element to the number of that element present in the compound (written in the form of subscript of the element in the molecular formula).
We add up all the resulting numbers and obtain the molecular mass in grams/mole.
Avogadro’s number is a proportionality factor that is used to relate the number of constituent particles in a sample with the amount of substance in that sample. Its SI unit is the mole inverse and is equal to . To calculate the mass of one molecule of a compound, we must have unit grams per molecule and our result would simply be the molecular mass divided by the number of atoms which is equal to Avogadro’s number.
The given compound is benzene with the molecular formula ${C_6}{H_6}$ . The constituent atoms are C and H with atomic masses $12$ and $1$ units respectively.
Atomic mass of Carbon (C) $= 12$
Number of atoms of Carbon $= 6$
Atomic mass of Hydrogen (H) $= 1$
Number of atoms of H $= 6$
Hence the molecular weight of ${C_6}{H_6} = \left( {6 \times 12} \right) + \left( {6 \times 1} \right)$
Molecular weight of benzene $= 72 + 6$
Molecular weight of benzene is $78g/mol$
The mass of one molecule of ${C_6}{H_6} = \dfrac{{78gmo{l^{ - 1}}}}{{6.022 \times {{10}^{23}}mo{l^{ - 1}}}}$
= $12.952 \times {10^{ - 23}}g$
Hence the molecular weight of ${C_6}{H_6}$ is $78g/mol$ and the mass of one molecule of ${C_6}{H_6}$ is $12.952 \times {10^{ - 23}}g$.

Note:
It must be noted that the rules given above for the molecular mass of a compound is applicable only for molecules non-ionic compounds only which carry no charge. Also, there are different names for molecular mass of a compound. The name may be molar mass, molecular weight, gram formula mass, etc but they all account for the mass of one mole of a substance in grams per mole. It must also be noted that sometimes the atomic masses of the elements are not given but we can obtain them from the periodic table.