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# Calculate the molar mass of the following substances(a). Ethyne, ${{\text{C}}_{2}}{{\text{H}}_{2}}$(b). Sulphur molecule, ${{\text{S}}_{8}}$(c). Phosphorus molecule, ${{\text{P}}_{4}}$(d). Nitric acid, $\text{HN}{{\text{O}}_{3}}$(e). Hydrochloric acid, $\text{HCl}$

Last updated date: 13th Jun 2024
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Hint: Memorize the atomic mass of every atom and then multiply the atomic mass with its stoichiometric coefficient and add it.

Complete Solution :
First, I would like you to know about the definition of atomic mass. It can be defined as-“The molar mass of any substance in grams per mole is numerically equal to the mass of that substance expressed in atomic mass units.”
I would like to explain it to you by giving an example.
For a molecule, (for example, ${{N}_{2}}$), the mass of a molecule is the sum of the atomic masses of the two nitrogen atoms. For Nitrogen, the mass of ${{N}_{2}}$ molecules is simply (14.01 + 14.01) = 28.02 amu or g/mol.
Here, We have shown the calculation of molar mass of each molecule:
(a). Ethyne, ${{\text{C}}_{2}}{{\text{H}}_{2}}$$= (12 \text{x 2) + (2 x 1) = 24 + 2 = 26} (b). Sulphur molecule, {{\text{S}}_{8}}$$ = (32$x$8) = 256$
(c). Phosphorus molecule, ${{\text{P}}_{4}}$$= (31 x 4) = 124 (d). Nitric acid, \text{HN}{{\text{O}}_{3}}$$ = \text{1 + 14 + (16 x 3) = 1 + 14 + 48 = 73}$
(e). Hydrochloric acid, $\text{HCl}$$=\text{1 + 35}\text{.5 = 36}\text{.5}$
Hence, the molar mass of the above compounds and molecules is.

${{\text{C}}_{2}}{{\text{H}}_{2}} = 28g/mol$
${{\text{S}}_{8}} = 256g/mol$
${{\text{P}}_{4}} = 124g/mol$
$\text{HN}{{\text{O}}_{3}} = 73g/mol$
$\text{HCl} = 36.5g/mol$
So, the correct answer is “Option A”.

The mole is the "counting unit" used by chemists to indicate the number of atoms, ions, molecules, or formula units present in a particular chemical sample. The mole is similar to other counting units that you've used before....pair (2), dozen (12), and gross (144). One mole of a compound contains Avogadro's number ($6.022\times {{10}^{23}}$ ) of molecules (molecular compound) or formula units (ionic compound). The molar mass of a compound tells you the mass of 1 mole of that substance. In other words, it tells you the number of grams per mole of a compound. The units for molar mass are, therefore, grams/mole.