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Home » Class 11 » Chemistry » Ionic Equilibrium » Ionisation of Weak Electrolytes

Ionisation of Weak Electrolytes

Last Updated on February 16, 2023 By Mrs Shilpi Nagpal

When acetic acid is dissolved in water, it dissociates partly into H+ and H3O+ and CH3COO‾ ions as:

CH3COOH + H2O sign of equilibrium reaction CH3COO‾ + H3O+

equilibrium constant when acetic acid is dissolved in water

In dilute solution , concentration of water is constant. The product of K and constant concentration of water

The product of K and concentration of water  is denoted by Ka, the ionization constant or dissociation constant of the acid.

Equilibrium constant when conc of water is constant

If C represents the initial concentration of the acid in moles L-1

and α , the degree of dissociation , then equilibrium concentration of the ions ( H3O+ and CH3COO‾ ) is equal to Cα and that of the undissociated acetic acid = C ( 1- α ) i.e. we have

Degree of dissociation of weak electrolyte

In case of weak electrolyte, The value of α is very small and can be neglected in comparison to 1 i.e. 1-α =1.Hence we get

α = √ Ka / C

If V is the volume of the solution in litres containing 1 mole of the electrolyte , C = 1/ V.Hence, we have,

α = √ Ka × V

For a weak base like NH4OH we have

α = √ Kb / C

α = √ Kb × V

For a weak electrolyte , the degree of ionisation is inversely proportional to the square root of molar concentration  or directly proportional to the square root of volume containing one mole of solute.This is called Ostwald’s dilution law.

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Filed Under: Chemistry, Class 11, Ionic Equilibrium Tagged With: degree of dissociation, ionization constant or dissociation constant of the acid, ka, kb, Ostwald's dilution law

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Comments

  1. Man Singh says

    September 10, 2019 at 5:08 pm

    Congrats Mrs Shilpi Nagpal, for preparing a very simple text to understand the ionization of weak electrolytes. Many topics also need your methodology to understand them. I do not know whether you could prefer to write simple text to understand the friccohesity.

  2. Darshan Patil says

    September 28, 2019 at 11:02 am

    This information helped me a lot for my assignments of college.Thanks for providing such good understandable method of Ostwalds dillution law.

  3. Marshal says

    April 1, 2020 at 6:21 pm

    Very nice………………… Thanks

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