Some basic concepts of chemistry,Part-3

Avogadro's hypothesis-

State that...

Equal volume of all gases under similar conditions of temperature and pressure contain equal number of atoms.

Example-Hydrogen   +    Chlorine = 2

                 1 vol.                  1 vol.       = 2 vol.

              N atoms             n atom      = 2n

            Molecules

    _on dividing throughout by 2n.

       1/2 atom             1/2 atom         = n molecules

This implies that one compound.

Atom of hydrogen chloride is made up of 1/2 atom of hydrogen and 1/2 atom of chlorine.

This is in direct conflict with Dalton's atomic theory which states that atoms are the ultimate particles of elements and are indivisible. This hypothesis was there fore rejected.

Avogadro's hypothesis-

It's states that equal volume of all gases under similar conditions of temperature and pressure contain equal number of molecules.

Applications of Avogadro's law.....

1.In the calculation of atomicity of Elementary gases:

Atomicity of an elementary substance is defined as the number of atoms of the elements present in 1 molecules of a substance.

Example-Atomicity of oxygen is 2 while that of ozone is 3 .

       

Hydrogen   +    Oxygen   ------>water

(Vapour)

2n molecules         n molecules    -- 2n

Molecules

On dividing throughout by 2n 

1 molecules         ½ molecules      1

Molecules

Thus one molecules of water contains ½ molecules of Oxygen. But 1molecules of water contains 1 atom of oxygen.

Hence ½ molecules= 

       1 molecules of oxygen =1 molecules of oxygen=1 atom of oxygen =2

2) To find the relationship between molecular mass and vapour density of a gas.

Molecular mass=2×vapour density

3)To find relationship between mass and volume of a gas.

22.4 litres of any gas at STP weight equal to the molecular mass of the gas expressed in grams.

This is called gram molecular volume law.

Atomic and molecular mass-

As an atom is small particles that it can't be seen or isolated,therefore it is impossible to determine the actual mass of a single atom by weighting it.

     The problem was finally solved by Avogadro's hypothesis.

If equal volume of two different gases are taken under Similar conditions of temperature and pressure and then weighted,the ratio of their masses will be equal to the ratio of their single molecule .

Thus, thought the actual masses of the atom could not be determined but their relative masses could be determined. If the atomic mass of the hydrogen is taken is 1, the relative atomic mass of oxygen is 16.

Initially the atomic masses of all the elements were obtained by elements were obtained by comparing with the mass of hydrogen taken as 1 but by doing. so, the atomic mass of most of the elements came out to be fractional.

        Therefore carbon is taken reference for the determination of atomic mass.

Atomic mass of elements is the number of times an atom of that elements is heaver than an atom of carbon taken as 12.

One atomic mass unit is equal to one twelvth of an atom of carbon isotope.

The atomic mass of an element is the average relative mass of its atom as compared with an atom of carbon 12 taken as 12.

   Fractional abundance of an isotope is the fraction of the total number of atoms that is comprised of that particular isotope. 

Gram Atomic mass-

The atomic mass of an element expressed in gram is called gram Atomic mass.

Example,

Atomic mass of oxygen is 16 amu.

Gram atomic mass of oxygen is 16 gm.

Molecular mass-

The molecular mass of a substance is the number of times the substance is heaver than one 12 th mass of an atom of carbon-12.

Example,

Mass of H=1u

Atomic mass of O =16 u

Molecular mass of water = 2× atomic mass of H+1× atomic mass of O 

=> 2×1+16×1

=>18u

Gram molecular mass-

The molecular mass of a substance expressed in gram is called gram molecular mass. 

Example, 

Molecular mass of oxygen=32u

Gram molecular mass of Oxygen = 32 gm.

 

          Dimensional Analysis------>

Any calculations involving the use of the  dimensions of the different physical quantities involved is called dimensional Analysis.

It's use for any one of following purposes .

 

1) To convert a physical quantity in one type of units into some other units:

The method is called-

    Factor level method or unit factor method.

a)First determine the unit conversion factor.

b) multiply the given physical quantity with the unit conversion factor, retaining the units conversion factor in such away that all units cancel out leaving behind only the atoms.

Next part will be coming soon .

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