Questions & Answers

Calculate the molar mass of the following substances
A.Ethyne, C$_2$H$_2$
B.Sulphur molecule, S$_8$
C.Phosphorus molecule, P$_4$ (Atomic mass of phosphorus = 31)
D.Hydrochloric acid, HCl
E.Nitric acid, HNO$_3$

Answer Verified Verified
Hint: The molar mass of molecules is defined as the summation of atomic masses of all the atoms present in a molecule. Its unit is represented in the term of grams. By knowing the atomic masses of elements, the molar mass can be determined.

Complete step by step answer:
Now, we know about the molar mass. As mentioned it is the total mass of the atoms in a molecule. So, we will calculate the molar mass of each part one by one.
-The first part (A) is Ethyne, the chemical formula is C$_2$H$_2$, we can see that it consists of carbon, and hydrogen. The atomic mass of carbon is 12, and hydrogen is 1. So, the molar mass of ethyne, 2$\times$12 + 2$\times$1 = 26 g.
-The second part (B) is sulphur molecule S$_8$, atomic mass of sulphur is 32. Thus, the molar mass of the sulphur molecule, 8$\times$32 = 256 g.
-The third part (C) is phosphorus molecule, atomic mass of phosphorus is 31. Thus, the molar mass of phosphorus molecule P$_4$, 4$\times$31= 124 g
-The fourth part (D) is hydrochloric acid (HCl), the atomic mass of H is 1, and Cl is 35.5. Thus, the molar mass of hydrochloric acid, 1+35.5 = 36.5 g
-The fifth part (E) is nitric acid (HNO$_3$), the atomic mass of H is 1, nitrogen is 14, and oxygen is 16. Thus, the molar mass of nitric acid, 1+14+3 $\times$ 16 = 63 g

Molar mass of (A) is 26 g, (B) is 256 g, (C) is 124 g, (D) is 36.5 g, (E) is 63 g.

The significance of atomic mass comes in solving the stoichiometric calculations and mole concept in chemistry. The unit of molar mass is g/mol. It acts as a bridge in calculating the number of moles because it is not possible to measure the number of moles directly. It should also be noted that the value of Avogadro’s number is $6.022\text{x1}{{\text{0}}^{23}}$$mo{{l}^{-1}}$ and is expressed as ${{N}_{A}}$. Also, The molecular weight is equal to the mass of one mole of the substance.