Question

Find the ratio of the mass of the elements present in the molecules of hydrogen sulphide ${{\text{H}}_{\text{2}}}{\text{S}}$ . Given that ${{\text{H}}_{\text{2}}}{\text{S}}$ molecular weight $= 34$ , atomicity $= 2$ , atomic weight $= 1$ .

Answer 1

The mass ratio or the percent composition by mass refers to the proportion of a particular molecule which consists of the constituent elements of that molecule. The percent composition by mass of a constituent element of a molecule can be calculated by the division of the mass of that element by the molar mass of the molecule, followed by its multiplication by 100.

Complete step by step answer:
According to the law of constant proportion, a pure compound, regardless of its source, is always made up of the same elements combined together in the same fixed proportion by weight. For example, water, regardless of its source, is always composed of hydrogen and oxygen combined together in the ratio of $1:8$ by weight.
The given molecule is a hydrogen sulphide molecule and its chemical formula is ${{\text{H}}_{\text{2}}}{\text{S}}$ . Thus, this molecule is having only one sulphur atom but there are two hydrogen atoms in it.
Let us determine the atomic mass of each element present in the molecule of hydrogen sulphide.
From the periodic table, it can be seen that the atomic masses of the element hydrogen is 1 and that of the element sulphur is 32.
Now, let us determine the mass of each individual element present in the hydrogen sulphide molecule.
The mass of an individual element is obtained by the multiplication of the atomicity or the number of atoms of that element with the atomic mass.
Atomicity of hydrogen is 2 and so the mass of hydrogen present is $2 \times 1 = 2$ .
Atomicity of sulphur is 1 and so the mass of sulphur present is $1 \times 32 = 32$ .
The molar mass of the molecule is given to be 34. Thus, the percent composition by mass of hydrogen is
$= \dfrac{2}{{34}} \times 100 \\\ = 5.88 \\\$
And, the percent composition by mass of sulphur is

$= \dfrac{{32}}{{34}} \times 100 \\\ = 94.12 \\\$

Hence, the ratio of the mass of the elements present in the molecules of hydrogen sulphide is $5.88:94.12$ .

Note:
Hydrogen sulphide can be prepared in the lab by the treatment of ferrous sulphide with a strong acid in a Kipp generator.
${\text{FeS + 2HCl}} \to {\text{FeC}}{{\text{l}}_{\text{2}}}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{S}}$
Hydrogen sulphide used in qualitative inorganic analysis is generated from thioacetamide.
${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}\left( {\text{S}} \right){\text{N}}{{\text{H}}_{\text{2}}} + {{\text{H}}_{\text{2}}}{\text{O}} \to {\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}\left( {\text{O}} \right){\text{N}}{{\text{H}}_{\text{2}}} + {{\text{H}}_{\text{2}}}{\text{S}}$
This gas is also produced when sulphur is heated with solid organic compounds.