Question

Find the atomicity of the following molecules
(a) ${\text{HCl}}$ (b)${\text{A}}{{\text{l}}_{\text{2}}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_{\text{3}}}$ (c) ${\text{Ca}}{\left( {{\text{N}}{{\text{O}}_{\text{3}}}} \right)_{\text{2}}}$ (d) ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$
Calculate the molecular mass of the following (atomic mass of Ca=$40$ u, S=$32$ u, Cl=$35$ u)
(a) ${\text{HN}}{{\text{O}}_3}$ (b) ${\text{CaC}}{{\text{l}}_2}$(c) ${\text{N}}{{\text{H}}_{\text{3}}}$ (d) ${\text{S}}{{\text{O}}_2}$

Answer 1

Hint: Atomicity is defined as the number of atoms of a molecule. Atomicity is determined by adding the number of atoms present in a molecule. The sum of the mass of each element of the compound will give the molecular mass.

Complete answer:
Atomicity represents the total number of atoms present in a molecule.
To calculate the atomicity we will take the sum of all numbers of atoms present in the molecule.
(a) ${\text{HCl}}$
The hydrogen chloride has one chlorine and one hydrogen atom so, its atomicity is,
$ = 1 + 1$
$ = 2$
So, the atomicity ${\text{HCl}}$ is $2$.
(b) ${\text{A}}{{\text{l}}_{\text{2}}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_{\text{3}}}$
The aluminium sulphate has two aluminium, three sulphur and twelve oxygen atoms so, its atomicity is,
$ = 2 + 3 + 12$
$ = 17$
So, the atomicity ${\text{A}}{{\text{l}}_{\text{2}}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_{\text{3}}}$ is $17$.
(c) ${\text{Ca}}{\left( {{\text{N}}{{\text{O}}_{\text{3}}}} \right)_{\text{2}}}$
The calcium nitrate has one calcium, two nitrogen and six oxygen atoms so, its atomicity is,
$ = 1 + 2 + 6$
$ = 9$
So, the atomicity ${\text{Ca}}{\left( {{\text{N}}{{\text{O}}_{\text{3}}}} \right)_{\text{2}}}$ is $9$.
(d) ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$
The hydrogen chloride has one chlorine and one hydrogen atom so, its atomicity is,
$ = 2 + 1 + 4$
$ = 7$
So, the atomicity ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ is $7$.
Molecular mass is the total mass of the molecules which is determined by adding the mass of each constituting atom.

Calculate the molecular mass of the following (atomic mass of Ca=$40$ u, S=$32$ u, Cl=$35$ u)
(a) ${\text{HN}}{{\text{O}}_3}$
Atomic mass of hydrogen is $1$ u, nitrogen is $14$ u and oxygen is $16$ u.
${\text{ = 1 + }}\,14{\text{ + }}\,\left( {16 \times 3} \right)$
$ = 63$
So, the molecular mass of ${\text{HN}}{{\text{O}}_3}$ is $63$.
 (b) ${\text{CaC}}{{\text{l}}_2}$
Atomic mass of calcium is $40$ u, and chlorine is $35$ u.
${\text{ = }}40{\text{ + }}\,\left( {35 \times 2} \right)$
$ = 110$
So, the molecular mass of ${\text{CaC}}{{\text{l}}_2}$ is $110$.
 (c) ${\text{N}}{{\text{H}}_{\text{3}}}$
Atomic mass of nitrogen is $14$ u, and hydrogen is $1$ u.
\[{\text{ = }}14{\text{ + }}\,\left( {1 \times 3} \right)\]
$ = 17$
So, the molecular mass of ${\text{N}}{{\text{H}}_{\text{3}}}$ is $17$.
 (d) ${\text{S}}{{\text{O}}_2}$
Atomic mass of sulphur is $32$ u, and oxygen is $16$ u.
\[{\text{ = }}32{\text{ + }}\,\left( {16 \times 2} \right)\]
$ = 64$
So, the molecular mass of ${\text{S}}{{\text{O}}_2}$ is $64$.

Note:
The representation of the element shows the atomicity. By looking at the molecule the atomicity can be determined. The subscript present after an atom in the molecule shows the number of that atom in the molecule. The subscript present after a bracket in the molecule shows the number of all atoms present in the molecule. Atomicity of elements are fixed but atomicity of molecules depends upon the constituting atoms. Diatomic molecules have two atoms. Polyatomic molecules have three or more atoms.