Chemistry

Atoms and Molecules

Gram Molecular Mass

The amount of a substance whose mass in grams is numerically equal to its molecular mass,is called gram molecular mass of that substance.In other words, the molecular mass of a substance expressed in grams is called its gram molecular mass. To write the gram molecular mass of a substance, we write its molecular mass and then replace the unit 'u' by the word 'gram or its symbol g .For example .
Molecular mass of oxygen,

O

₂ = 32 u
So,Gram molecular mass of oxygen,

O

₂ = 32 grams (or 32 g)
The gram molecular mass of a substance represents the mass of 1 mole of molecules ( $0.022X10$ ²³ molecules) of that substance. So, the number of molecules present in 1 gram molecular mass of any substance is

0.022X10

²³ molecules. The molecular mass of hydrogen (

H

₂) is 2 u, so the gram molecular mass of hydrogen (

H

₂) is 2 grams; the molecular mass of nitrogen (

N

₂) is 28 u, so the gram molecular mass of nitrogen (

N

₂) is 28 grams; the molecular mass of oxygen (

O

₂) is 32 u, so the gram molecular mass of oxygen (

O

₂) is 32 grams ; and the molecular mass of water (

H

₂O) is 18 u, so the gram molecular mass of water (

H

₂O) is 18 grams.
As discussed above, we can also use the term 'molar mass of the substance in place of 'gram molecular mass' of substance.The molar mass of a substance is the mass of 1 mole of its molecules. The molar mass of a substance has $0.022X10$ ²³ molecules of the substance in it.The molar mass of a molecular substance is equal to the molecular mass of the substance expressed in the units of g/mol. The molecular mass of hydrogen (

H

₂) is 2 u, so the molar mass of hydrogen (

H

₂) is 2 g/mol ;the molecular mass of nitrogen (

N

₂) is 28 u, so the molar mass of nitrogen (

N

₂) is 28 g/mol ; the molecular mass of oxygen (

O

₂) is 32 u, so the molar mass of oxygen (

O

₂) is 32 g/ mol; and the molecular mass of water (

H

₂O) is 18 u, so the molar mass of water (

H

₂O) is 18 g/mol.
Please note that if an element exists as molecules (such as hydrogen,nitrogen, oxygen, chlorine, etc.), then it can have gram atomic mass as well as gram molecular mass. But if an element does not exist as molecules (such as carbon, sodium, calcium, magnesium, etc.), then it can have only gram atomic mass, it cannot have gram molecular mass.All the compounds, however, have only gram molecular mass.From the above discussion we conclude that:
(i)'Gram atomic mass of an element' and 'Molar mass of an element'are just the same, and
(ii) 'Gram molecular mass of a substance' and "Molar mass of a substance are also just the same.
We will now solve one problem based on the calculation of molar mass.
Sample Problem. Calculate the molar masses of the following substances :
(a) Ethyne,

C

₂H₂ (b) Sulphur molecule,

S

₈ (c) Phosphorus molecule,

P

₄ (d) Hydrochloric acid, HCl
(e) Nitric acid,

HNO

₃
(Atomic masses :C = 12u;H = lu;S = 32u;P = 31u;Cl = 35.5 u ; N = 14 u ; O = 16 u)
Solution. The molar masses of all these substances will be equal to the respective molecular masses expressed in g/mol. Now :
(a) Molar mass of ethyne,

C

₂H₂ = Mass of C x 2 + Mass of H x 2
= 12 x 2 + 1 x 2
= 24 + 2
= 26 g/mol
(b) Molar mass of sulphur molecule,

S

₈ = Mass of S x 8
= 32 x 8
= 256 g/mol
(c) Molar mass of phosphorus molecule,

P

₄ = Mass of P x 4
= 31 x 4
= 124 g/mol
(d) Molar mass of hydrochloric acid, HCl = Mass of H + Mass of Cl
= 1 + 35.5
= 36.5 g/mol
(e) Molar mass of nitric acid,

HNO

₃ =Mass of H + Mass of N + Mass of O x 3
= 1 + 14 + 16 x 3
= 15 + 48
= 63 g/mol

Notes

Atoms and Molecules - Notes

1. Drawbacks of Dalton’s Atomic Theory