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- by ♪⚝Sanjiv❀Jaiswal❁⩸ 8 months ago

↪ It is deduced from Avogadro law that 1 g molecular mass of each of the ideal gases occupies same volume at NTP which is 22.4 L.

↪ This volume is molar volume.

↪ Avogadro hypothesis states that equal volumes of all the gases contain same number of molecules at the same temperature and pressure.

↪ A molar volume contains same number of molecules and this constant number is Avogadro number.

⁕ Deduction of Avogadro number

Consider H_{2} gas to deduce the number.

Molecular mass of H_{2} = 2 and 1 g molecular mass of H_{2} = 2 g.

Let w - mass of H atom and 2w is the mass of H_{2} molecule.

i.e. 2 wg contain 1 molecules of H_{2}

2 g contain 1/w molecules of H_{2}

i.e., 1 g mol of H_{2} contains 1/w molecules of H_{2} and it is constant number not only for H_{2} but for all gases.

This constant number of molecules is Avogadro number, which is calculated as,

i.e., molar volume of 1 g mol of any gas contains Avogadro number of molecules

6.023 x 10^{23} molecules.

⁕ Molecular of a Compound from its Volumetric Composition

↪ The molecular formula of some compound gases can be deduced from the law.

Let us deduce the formula of steam.

Consider: Hydrogen + Oxygen = Steam

It is observed by experiment that

↪ 2 volumes of Hydrogen + 1 volume of Oxygen = 2 volumes of steam

Let, 1 Volume of gas contain n molecules of it. Then by Avogadro's hypothesis:

2n molecules of H_{2} + n molecules of O_{2} = 2n molecules of steam

or, 1 molecule of H_{2} + 1/2 molecule of O_{2} = 1 molecule of steam

It has been deduced with reference to hydrogen that elementary gases are diatomic.

↪ 1 molecule of hydrogen contains 2 atoms of hydrogen and 1/2 molecule of oxygen contains 1 atom of oxygen.

Therefore, simple formula for steam is H_{2}O and the molecular formula (H_{2}O)_{n} where n is small integer.

The vapor density of steam is 9, hence its molecular weight is 18.

Therefore, for (H_{2}O)_{n} ⇒ (2+16)_{n} = 18 and n=1.

Hence, molecular formula for steam is H_{2}O.

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