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The number of atoms in a 12 g sample of carbon - 12 is called Avogadro's  number. Avogradro's number is represented by NA. The value of Avogadro's number is 6.022 × 1023. A mole of a substance contains Avogadro's number of molecule.

2. What is molar mass of a substance?

The average mass of one mole of any material is called its molar mass or it is the mass of one mole of the substance. Since the unit of mass is kg or g and that of n is mol, hence the unit of molar mass is gram per mole (g/mol) or kg/mol.

3. Calculate the number of atoms in (i) 52 moles of He.  (ii) 52u of He (iii) 52g of He.

(i) 1 mol of He contains 6.022 × 022 × 1023 He atom.
52 mol of He contains  6.022 × 1023 × 52  He atom
= 3.312 × 1025 He atom.
(ii)  1 molecule of He   =  4 u (at0omic mass of He = 4)
Therefore,  4 u  = 1 He atom = 13 He atom
(iii) 1 mol of  He  =  4g = 6. 022 × 1023 He atom
4 g of He contains   = 6. 022 × 1023 He atom
52 g He contains 6.022 × 1023 × 52/4 He atom
= 7. 83 × 1024 He atom

4. How much mass (in gram units) is represented by the following.
(a) 0. 2 mol of NH3   (b) 3. 0 mol of CO2

(a) Molar mass of NH3 = (1 × 14. o× 3 × 1.0) g/mol
= (14 + 3) g/mol = 17 g/mol.
Then, mass of 0.2 mol of NH3  = 0. 2 mol ×17 g/mol
= (0.2 × 17) g
= 3.4 g
(b)  Molar mass of  CO =  (1× 12.0 + 2 × 16.0) g/mol
= (12 + 32) g/mol = 44g/mol
Then, mass of 3.0 mol of CO2 =  3. 0 mol × 44 g/mol
= 132 g.
5. Discuss mole concept.

The mole is a concept of quantity in terms of number and mass, which relates the mass of a material to the number of atoms, ions or molecules in it. The mole is the amount of a substance which contains the same number of chemical units (atoms, molecular or ions) as there are atoms in exactly 12 grams of pure carbon - 12. Thus a mole represents a collection of 6.022 × 1023 chemical units (atoms, molecules or units). The number 6.022 × 1023 is called the Avogadro's number.

6. What is empirical formula?

The simplest of a substance which gives the relative number of atoms of each element present in the molecule of that substance is called as the empirical formula. For example, a compound contains carbon, hydrogen and oxygen in the ratio 1 : 2 : 1, its empirical formula will be CH2O

7. How is molecular formula of a compound related to its empirical formula?

Molecular formula    =  n × Empirical formula
= (Empirical formula)n.
Where n may be 1,2,3.....
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