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

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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

অঙ্গনওয়াড়ি কর্মী ও সহায়িকা প্র্যাকটিস সেট ৯

অঙ্গনওয়াড়ি পরীক্ষায় পাটিগণিত, পুষ্টি, জনস্বাস্থ্য ও মহিলাদের অবস্থা, ইংরেজি, সাধারণ জ্ঞান ইত্যাদি বিষয়ে প্রশ্ন আসে। পূর্ববর্তী সময়ে এই পরীক্ষায় প্রধানত MCQ ধাঁচের প্রশ্ন ছিল। আজকের প্রস্তুতি সেটে ১০টি গুরুত্বপূর্ণ MCQ ধরণের প্রশ্ন ও উত্তর সন্নিবেশিত করা হয়েছে, যা পরীক্ষার জন্য বিশেষ প্রাসঙ্গিক।

1 / 10

1. পোলিও রোগটি ঘটে কার দ্বারা?

2 / 10

7. কুটির পনির থেকে তৈরি এবং প্রায় উৎসব অনুষ্ঠানের সাথে যুক্ত ঐতিহ্যবাহী বাঙালি মিষ্টি কী?

3 / 10

2. ICDS -এর ধারণাটি কোথা থেকে এসেছে?

4 / 10

9. পেলেগ্ৰা রোগ হয় কোন্ ভিটামিনের অভাবে?

5 / 10

8. ভারতবর্ষে কবে “National Forest Policy” চালু হয়?

6 / 10

10. যে কোনো প্রকার ডিমে থাকা ওমেগা 3 শরীরের কোন অঙ্গকে কার্যকর রাখতে সাহায্য করে?

7 / 10

4. You should abide _____ the rules.

8 / 10

5. মানবদেহে পরিপাকনালীর খাদ্য শোষণকারী অংশটি হল—

9 / 10

6. একটি একমুখ খোলা লম্ববৃত্তাকার শঙ্কুর কয়টি তল?

10 / 10

3. যক্ষা রোগের জীবানু সবচেয়ে বেশি আক্রমণ করে—

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