Molecular Mass

Every object that has space and occupies is called matter. Through this definition we can get an idea about how important is mass for the identity of a substance. The mass of any object whether small or large, can categorise it into matter. Just like atoms have atomic mass, similarly molecules have molecular mass.

We know that molecules are those substances which are made up of two or more than two atoms chemically combined together and are capable of independent existence.So, to calculate molecular mass we need to know the mass of each element whose atoms comprise the molecule and also number of atoms of that element.

Actual masses of atoms of the elements are very, very small. For example, one atom of hydrogen has mass of 1.673 x 10^-24 grams. It is not convenient to use such small complicated figures in our calculations, therefore, it was necessary to define atomic masses in such a way that we get simple figures for them.

So, a convention, to reduce such large figures, was introduced. The use of carbon-12 was suggested to make it easier to express mass of elements. The carbon-12 element was assigned to have a mass of 12 u. ‘u’ is the unit of atomic mass. The mass of all the other elements can now be expressed in terms of mass of carbon-12 element. The formula to find relative atomic masses of elements in terms of mass of 1 atom of carbon-12 is:

Atomic mass unit = 1/12 x (mass of a carbon-12 atom)

Or 1u = 1/12 x (mass of carbon-12 atom)

Thus, 1 atomic mass unit is expressed as exactly 1/12 mass of carbon-12.

This convention is now used to express masses in whole chemistry. So, we calculate molecular masses in the same convention.

Just as an atom has atomic mass, in the same way, a molecule has a molecular mass. the molecular mass of a substance is the relative mass of its molecule as compared with the mass of a carbon-12 atom taken as 12 units. The molecular mass of a substance indicates the number of times one molecule of the substance is heavier than 1/12 (one-twelfth) of a carbon-12 atom. For example, the molecular mass of hydrogen is 2, which means that a molecule of hydrogen is 2 times heavier than 1/12 of a carbon-12 atom. The molecular mass is also expressed in atomic mass unit (u).

If the molecular formula of a substance is known, its molecular mass can be calculated, because the molecular mass is equal to sum of the atomic masses of all the elements present in one molecule of the substance. For example, one molecule of water (H₂O) contains 2 atoms of hydrogen and 1 oxygen. So, the molecular mass of water will be equal to the sum of the masses of 2 hydrogen atoms and 1 oxygen atom. Knowing that the atomic mass of hydrogen is 1 u and that of oxygen is 16 u, the molecular mass of water can be calculated as follows:

Mass of H atom = 1 u

Mass of 2 H atoms = 1 x 2u = 2 u

Mass of O atom = 16 u

Now, molecular mass of H₂O = mass of 2H atoms + mass of O atom

= 2+16 u = 18 u

Thus, the molecular mass of water is 18 u.

In case of ionic compounds like sodium chloride which consist of ions and not molecules the term formula mass is sometimes used in place of molecular mass.

So, to calculate the molecular mass of compounds we need to multiply the mass of each element with its subscript.

Questions about Molecular Mass from NCERT textbook – Page 40

Question 1:-

Calculate the molecular masses of H₂ , O₂ , Cl₂ , CO₂ , CH₄ , C₂H₆ , C₂H₄  , NH₃ , CH₃OH. (Atomic masses : H = 1; O = 16; Cl = 35.5; C = 12; N = 14)

Answer:-

• Molecular mass of H₂= 2 x 1 = 2u
• Molecular mass of O₂= 2 x 16 = 32u
• Molecular mass of Cl₂ = 2 x 35.5 = 71u
• Molecular mass of CO₂ = (12 + 2×16) = 44u
• Molecular mass of CH₄ = (12 + 4×1) = 16u
• Molecular mass of C₂H₆ = (2×12 + 6×1) = 30u
• Molecular mass of C₂H₄ = (2×12 + 4×1) = 28u
• Molecular mass of NH₃ = (14 + 3×1) = 17u
• Molecular mass of CH₃OH = (12 + 3×1 + 16 + 1) = 32u

Question 2:-

Calculate the formula unit masses of ZnO , Na₂O , K₂CO₃. (Given : Atomic masses of Zn = 65u; Na = 23u; K = 39u; C = 12u and O = 16u)

Answer:-

• Formula mass of ZnO = (65 + 16) = 81u
• Formula mass of Na₂O = (2×23 + 16) = (46 + 16) = 62u
• Formula mass of K₂CO₃ = (2×39 + 12 + 3×16) = (78 + 12 + 48) = 138u

Conclusion – Molecular mass of a compound is expressed in atomic mass units (u). It is the mass of a molecule compared to the mass of carbon-12 atom. The molecular mass is calculated by adding the mass of all the atom of elements. For this we need the mass as well as number of atoms of elements present in that compound. To calculate the molecular mass of molecules we multiply the number of atoms by their atomic mass.