Definations and molecular mass calculation

Day 1         4th period
21. 4.20

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GOOD MORNING BOYS,
Today we will be going through the various definations and calculate the molecular masses of compound.

Learning objective :
students will be able to

• Define atomic mass, Molecular Mass & formula mass
• calculate molecular masses of compounds.

Atomic mass unit 
It is defined as a mass exactly equal to one twelfth the mass of one carbon – 12 atom.

Gram atomic mass of a substance –
 The atomic mass of a substance when expressed in grams is known as its gram atomic mass.
The atomic mass of Sulphur is 32u. Gram atomic mass of Sulphur is 32g.
 Molecular mass
It is the sum of atomic masses of the elements present in a molecule.
 It is obtained by multiplying the atomic mass of each element by the number of its atoms and  adding them together.
 Molecular mass expressed in grams is called gram molecular mass.

Q1. Find out the value of molecular weight of the given compounds:

(i) $CH_{4}$      (ii)$H_{2}O$      (iii)$CO_{2}$

Ans.

(i)$CH_{4}$ :

Molecular weight of methane, $CH_{4}$

= (1 x Atomic weight of carbon) + (4 x Atomic weight of hydrogen)

= [1(12 u) +4 (1u)]

= 12 u + 4 u

= 16 u

(ii) $H_{2}O$ :

Molecular weight of water, $H_{2}O$

= (2 x Atomic weight of hydrogen) + (1 x Atomic weight of oxygen)

= [2(1) + 1(16 u)]

= 2 u +16 u

= 18 u 

(iii) $CO_{2}$ :

= Molecular weight of carbon dioxide, $CO_{2}$

= (1 x Atomic weight of carbon) + (2 x Atomic weight of oxygen)

= [1(12 u) + 2(16 u)]

= 12 u +32 u 

= 44 u

(iv)  HNO3 

Atomic mass of H =1u

Atomic mass of N =14u

Atomic mass of O =16u

Molecular mass of HNO3 = 1 + 14 + (16 x 3) = 63u

Formula Mass: 
Sum of atomic masses of all atoms in a formula unit of the compound. Applicable for Ionic Compounds.

For example: - Formula mass of sodium chloride (NaCl) = atomic mass of sodium (Na) + atomic mass of chlorine (Cl) = 23.0 u + 35.5 u = 58.5 u.

Molar mass

The mass of one mole of a substance in grams is called its molar mass.
The molar mass in grams is numerically equal to atomic /molecular/formula mass in u.
Molar mass of oxygen (O) =16 g.
Molar mass of water (H2O) = 18 g.

Assignment Questions :

Calculate the molecular mass of following compound :

1. sulphuric acid

2. Glucose

3. Ammonia