pH Scale of Acid and Bases: Formula and Equation
Introduction and Explanation:
The ionization of chemicals takes place mostly in an aqueous medium. Arrhenius’s concept plays an important role in the ionization of acids and bases. The strength of acids and bases is classified on the basis of Arrhenius’s concept.
Strong acids and bases are chemicals that dissociate completely in an aqueous medium.
- Strong Acid
- Strong Base
Weak acids and bases are chemicals that dissociate completely in an aqueous medium.
- Weak Acid
- Weak Base
Classification of Solutions
On the basis of their relative values, solutions can be classified as acidic, basic, and neutral.
- [H3O+] > [OH–] => ACIDIC
- [H3O+] > [OH–] => BASIC
- [H3O+] > [OH–] => NEUTRAL
The pH Scale
The concentration of hydronium ion is represented in the form of molarity. It is expressed on a logarithmic scale. It is defined as the negative logarithm to base 10 of the activity of hydrogen ions.
It is represented as:
pH = -log aH+
= -log {[H+] / mol L-1}
Calculation of pH of HCl
- The HCl solution has a concentration of hydronium ion 10 -2 M.
pH = -log aH+ = -log {[H+] / mol L-1}
= -log [10 -2]
= 2
Calculation of pH of NaOH
- NaOH solution has a concentration of hydronium ion 10 -10 M and hydroxide ion is 10 -4 M.
pH = -log aH+ = -log {[H+] / mol L-1}
= -log [10 -10]
= 10
Calculation of pH of Pure Water
Pure water has a concentration of hydronium ion 10 -7 M.
pH = -log aH+ = -log {[H+] / mol L-1}
= -log [10 -7]
= 7
The pH Values
Acidic solutions contain a concentration of hydronium ions greater than 10 -7 M. Basic solutions contain a concentration of hydronium ions lesser than 10 -7 M. Neutral solutions contain a concentration of hydronium ions equal to 10 -7 M.
Dissociation Constant of Water
According to the reaction H2O + H2O → H3O+ + OH–. The dissociation constant is written as:
K = [H3O+][OH–]/ H2O
Where H2O is pure, so its concentration is constant. Therefore, the equilibrium constant is now written as:
Kw = [H3O+][OH–] —————à equation (i)
Dissociation Constant of Water
Let us take negative logarithm on both sides of equation (i)
-log Kw = -log {[H3O+][OH–]}
= – log [H3O+] – log [OH–]
For pure water [H3O+] = [OH–] = 10 -7
Therefore
-log Kw = – log [10 -7] – log [10 -7]
= 7 + 7
= 14
Dissociation Constant of Water
log Kw = – log [H3O+] – log [OH–]
pKw = pH + pOH
= 14
Measurement of pH
It is important to know the pH value whenever we are dealing with biological and cosmetic applications. Found with the help of pH paper ranges from 0 to 14. For higher accuracy pH meters are used.
Summary
Acidic, Basic, and Neutral
On the basis of their relative values, solutions can be classified as acidic, basic, and neutral.
- [H3O+] > [OH–] => ACIDIC
- [H3O+] > [OH–] => BASIC
- [H3O+] > [OH–] => NEUTRA
The pH Scale
- The concentration of hydronium ion is represented in the form of molarity.
- It is expressed on a logarithmic scale
- Defined as the negative logarithm to base 10 of the activity of hydrogen ion.
- It is represented as pH = -log aH+
= -log {[H+] / mol L-1}
Measurement of pH
The pH Values
- Acidic solutions [H3O+] > 10 -7 M
- Basic solutions [H3O+] < 10 -7 M
- Neutral solutions [H3O+] = 10 -7 M
Dissociation Constant of Water
Kw = [H3O+][OH–]
-log Kw = -log {[H3O+][OH–]}
= – log [H3O+] – log [OH–]
pKw = pH + pOH
= 14
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