Atomic Mass Unit, Mole Concept & Importance of Avogadro’s Number – Lecture 01

Foundation Course on Chemistry for WBCS”24 & 25 (Prelims & Mains)

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Mole Concept

  • It is not practically possible to find the mass of a minute particle like an atom, a molecule, or an ion etc. Therefore, a collection of 6.022×1023 elementary particles called Mole is taken for all practical purposes.

Mole is simply like a dozen or a gross. A dozen is a collection of 12 objects, a gross is a collection of 144 objects, similarly a mole is a collection of 6.022×1023 particles (atoms or molecules or ions).

A mole is the amount of pure substance containing the same number of chemical units as there are atoms in exactly 12 grams of carbon -12.

Note:

When the particle is not specifically mentioned, then one mole is taken as 6.023 x 1023 particles or molecule.
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  • Avogadro’s number is defined as the number of atom present in 12 g (gram atomic mass) of C-12 isotope i.e., 6.022×1023 atoms .

OR

  • Avogadro’s number is the number of elementary units, i.e., atom, ions or molecules present in one more of a substance .
  • It is denoted by NA
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Mole of Atoms

  • “one mole of atoms contain 6.022×1023 atoms having mass equal to its gram atomic mass” .
  • Thus, one mole of oxygen atoms contain 6.022×1023 atoms of oxygen and weighs 16 g.

Mole of Molecules

  • One mole contain 6.022×1023 molecules and is equivalent to the gram molecular mass of atom given substance. Thus 1 mole of 02 contain 6.022×1023 molecules and weighs 32 g.
  • 1 mole of water contain 6.022×1023 molecules and weighs 18 g .
  • Mole and molar volume: It has been notices that one mole of any gaseous molecule occupied 22.4 dm3 (litre) or 22400 cm3 (ml) at S.T.P. The volume is known as molar volume.
  • The molar volume of a gas can be defined as the volume occupied by one mole of a gas as S.T.P.
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Vapour Density and its Relation to Relative Molecular Mass

The relative Vapour Density of a gas (or a vapour) is :

The ratio between the masses of equal volumes of gas (or vapour) and hydrogen under the same conditions of temperature and pressure.

Relative V.D =
mass of volume "v' of the gas under similar conditions 
mass of volume 'v' of hydrogen gas under similar conditions

According to Avogadro’s Law volumes at the same temperature and pressure may be substituted by molecules. Hence,

Relative V.D. =
Mass of 1 molecule of gas or Vapour
Mass of 2 atoms of Hydrogen

(Molecule of hydrogen contains 2 atoms) By multiplying both sided by 2

2 x Rel. V.D. =
Mass of 1 molecule of gas or Vapour
Mass of 1 atoms of Hydrogen
2 x Rel. V.D = Rel. molecular mass of a gas or vapour.

The relative molecular mass of a gas or vapour is twice its vapour density.

For example, the relative vapour density of gaseous chlorine is 35.5, hence its relative molecular mass would be 2 x 35.5 or 71.0
It is found by direct weighing that 1 dm3 (or 1 L) of hydrogen at S.T.P weighs 0.089 g. Therefore, 1 dm of any gas, whose vapour density at S.T.P. is D, would weigh D X 0.089 g at S.T.P.

OR

D X 0.089 g of any gas occupies 1 dma (1 L)

D go of any gas will occupy
1
0.089

or

11.2 dm3 or 11.2 L

Or 2D of any gas will occupy 22.4 dm3 or 22.4 L.

But, 2 x V.D. = Molecular mass .
Thus, gram molecular mass of any gas occupy 22.4 litres at S.T.P •, which contains 6.022×1023 molecules of that gas and is equal to 2 mole that gas.
The molar volume of a gas is the volume occupited by one gram – molecular mass simple by one mole of the gas at S.T.P. It is equal to 22.4 dm3.

  1. The number of atoms in one mole (one g-atom of an element) is 6×1023
  • Calculate,
    (a) The number of molecules in 14 g of nitrogen gas.
    (b) The total number of atoms in 18 g of water.
    (c) The number of chloride ions in 111 g of anhydrous calcium chloride

Atomic Mass Unit, Mole Concept & Importance of Avogadro’s Number – Lecture 01 Quiz

Atomic Mass Unit, Mole Concept & Importance of Avogadro’s Number – Lecture 01 Quiz

1 / 10

10. How many moles are in 24 grams of magnesium (Mg)?

2 / 10

9. Which unit is used to measure the amount of a substance in chemistry?

3 / 10

8. The mole concept is essential in:

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7. Which of the following is NOT true about Avogadro's number?

5 / 10

6. How many grams are in one mole of carbon (C)?

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5. The mass of one mole of a substance is called:

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4. Which concept helps in relating the mass of a substance to the number of atoms or molecules?

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3. What is Avogadro's number?

9 / 10

2. How many particles are present in one mole of a substance?

10 / 10

1. What is the basic unit for measuring atomic mass?

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