Question

The correct increasing order of molecular weights is:
(A) \[{H_2}O > {H_2}S > C{O_2} > S{O_2}\]
(B) \[{H_2}O > {H_2}S < C{O_2} > S{O_2}\]
(C) \[{H_2}O < {H_2}S < C{O_2} < S{O_2}\]
(D) \[{H_2}O > {H_2}S > C{O_2} < S{O_2}\]

Answer 1

Hint: Atomic masses of the following elements are: oxygen = 16 g \[mo{l^{ - 1}}\], carbon = 12 g \[mo{l^{ - 1}}\], sulphur = 32 g \[mo{l^{ - 1}}\], hydrogen = 1 g \[mo{l^{ - 1}}\]. Many times, this data would not be provided in a question, so it is a good practice to memorise the atomic numbers and masses of at least the first 30 elements.

Complete Step-by-Step Solution:
Molecular formulas can be calculated as the sum of the atomic masses multiplied by the number of corresponding atoms present of each constituent element in the compound.
i.e. mol. weight of given compound = [(no. of atoms of element 01) (mol. weight of element 01) + [(no. of atoms of element 02) (mol. weight of element 02) + …]
Let us calculate the molecular weight for the given compounds, i.e. water, hydrogen sulphide, carbon dioxide and sulphur dioxide:
Water = \[{H_2}O\] = 2 (mol. weight of hydrogen) + 1 (mol. weight of oxygen)
= 2 (1) + 1 (16)
= 2 + 16 = 18 g \[mo{l^{ - 1}}\]
Hydrogen sulphide = \[{H_2}S\] = 2 (mol. weight of hydrogen) + 1 (mol. weight of oxygen)
= 2 (1) + 1 (32)
= 2 + 32 = 34 g \[mo{l^{ - 1}}\]
Carbon dioxide = \[C{O_2}\] = 1 (mol. weight of carbon) + 2 (mol. weight of oxygen)
= 1 (12) + 2 (16)
= 12 + 32 = 44 g \[mo{l^{ - 1}}\]
Sulphur dioxide = \[S{O_2}\] = 1 (mol. weight of sulphur) + 2 (mol. weight of oxygen)
= 1 (32) + 2 (16)
= 22 + 32 = 64 g \[mo{l^{ - 1}}\]
Hence the correct order of increasing order of molecular weights of the given compounds is:
\[{H_2}O > {H_2}S > C{O_2} > S{O_2}\]

Hence, Option A is the correct option.

Note: Molecular weight, also called molecular mass, mass of a molecule of a substance, based on 12 as the atomic weight of carbon-12. It is calculated in practice by summing the atomic weights of the atoms making up the substance's molecular formula.