What is Law of Mass Action? derive Henderson- Hasselbalch equation?

Law of Mass Action:

For a reversible reaction at equilibrium, the rate of a chemical reaction at constant temperature is proportional to the products of the concentrations of each reacting substance. 

derive Henderson- Hasselbalch equation:

 According to the bronsted lowry theory of acid and bases, an acid is capable of donating a Proton and a base is capable of accepting a Proton . After the acid has lost its proton, it is said to exist as the conjugated base. similarly a protonated base is said to exist as the conjugated  acid. 

For example,

HA -----H+ + A-

Now apply the law of mass action, 

Ka  = [H+] + [A-] / [HA]

or,[H+] = Ka  X  [HA]/ [A-]

we can write it,

[H+]= Ka  X  [Acid]/[Salt]

Taking - log of both side of the equation:

-log [H+]=-log Ka  -log [Acid]/[Salt]

But,   

-log [H+]=pH And,  -log Ka  =pKa

So, pH = pKa -log [Acid]/[Salt]

or, pH = pKa + log [Salt] /[Acid]

for basic solution, henderson hasselbalch equation will be,

pOH = pKb + log [Salt] /[Base]

This relationship is called the henderson hasselbalch equation.