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Acids And Bases: Ionisation And pH

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Acids And Bases: Ionisation And pH - Lesson Summary

According to the Arrhenius theory the strength of an acid and a base is related to the ease with which they function as a source of H+ ions and OH- ions in an aqueous medium.

According to Bronsted - Lowry theory a strong acid readily donates the proton and a strong base readily accepts a proton.

The strength of an acid is the tendency of an acid to donate protons and strength of base is the tendency to accept protons.

Strong acids have weak conjugate bases and weak acids have strong conjugate bases.

The ionic product of water (Kw) at a given temperature is defined as the product of the concentrations of H+ and OH- ions in water (or) in aqueous solutions.

Kw = [H+][OH-]
[H+] = 1.0 x 10-7 mole/litre
[H+] = [OH-] = 1.0x10-7 mole/litre
Kw = [H+][OH-] = (1.0x10-7 mole/litre)2 at 298K
Kw = [H+][OH-] = 1.0x10-14 mole2/litre2 at 298K
Kw = 1.0x10-14 mole2/litre2

Aqueous solutions can be categorised as neutral solutions, acidic solutions and basic solutions, depending upon the relative number of H +and OH ions.

In neutral sollution [H+] = [OH-]
In acidic solutions [H+] > [OH-]
In basic solutions [H+] < [OH-]

The pH of a solution is defined as negative logarithm to the base 10 of the molar hydrogen concentration in it.

pH = -log10[H+]

Ex:

H+ ion concentration in water is 10-7, the pH of water is:

pH = -log10[H+]

pH = -log10[10-7]

pH =-(-7)

pH =7

Therefore pH of water is 7.

Based on the value of the ionic product of water the pH scale has values from 1 to 14.

For neutral sollution [H+] = [OH-] and pH = 7
For acidic solutions [H+] > [OH-] and pH < 7
For basic solutions [H+] < [OH-] and pH > 7

Relationship between pH and pOH,

- log Kw = -log{[H+][OH-]} = -log(1.0x10-14)
              = {-log[H+]}+{-log[OH-]} = -log(1.0x10-14)
pKw = pH + pOH = 14

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