Question

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, $\text{HCl}$

Answer 1

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”.

Additional Information:
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.

Note: We should know that Molar mass is useful in determining results of experiments. It is also used to determine what percentage any given element in a compound contributes to the total mass of the compound and never forget to write the unit after calculation of molar mass of any compound or molecule. The unit of molar mass is g/mol.