LAW OF MASS ACTION.

• Rate of any chemical reaction at any instant, at a given temperature is directly proportional to the product of molar concentrations of reactants.
• It is the mathematical expression of the law of chemical equilibrium.

Suppose 'A' react with 'B' to produce 'C' and 'D' and reversely 'C' react with 'D' to produce 'A' and 'B'.
Velocity of forward reaction, say V1 and that of backward reaction, say V2.

Now according to the law of mass action:

 

 At equilibrium:

 

 

Here "K" is the measure of chemical affinity of the reactants. If "K" is large, A' and 'B' will show more tendency to produce 'C' and 'D' compared to the tendency of 'C' and 'D' to produce 'A' and ,B'. For highly
dissociable (ionizable) substance "K" value will be high and for less dissociable substance "K" value will
be low. "K" value of a particular reaction changes with temperature and it is definite for a given
chemical reaction at a given temperature.

 Importance of the law of mass action:

1. It explains the basis of biochemical reaction and predicts the reaction rate.
2. It expresses the concept of pH.
3. It explains the concept of solubility product of sparingly soluble salts in water.

What is HENDERSON-HASSELBALCH EQUATION? What is the importance of it?

Henderson Hasselbalch equation:

It is the equation proposed to explain the relationship between the pH of a weak acid in solution with
its dissociation constant.

In case of a buffer solution composed of weak acid, (e.g. H2CO3) and its salt (e.g. NaHCO3), the
concentration of HCO3 is equivalent to the concentration or NaHCO3, because NaHCO3, is almost totally
dissociated to HCO3 whereas there is negligible dissociation of H2CO3, to produce HCO3 since it is a
weak acid' So the free HCO3 present in this buffer solution is almost totally derived from the NaHCO3".

Importance of Henderson-Hasselbalch equation:

1. It is used to determine the pH of weak acid.
2. It is used to determine the pH of buffer solution.
3. It helps to quantify the buffer components needed to prepare a buffer solution of definite pH.
4. It helps in the determination of blood pH and evaluation of the acid base status of patients.
5. It is used lo assess the internal consistency of acid base parameters of blood (pH, PCO2, and HCO3 measured clinically for correct interpretation and diagnosis.
6. It is used to predict the bio-availability of drugs by calculating the abundance of ionic forms of acidic or basic drugs at a particular pH.