[Sunil Simha]

This was a derivation found in my textbook regarding the dependence of pH on solubility of salts

Consider a salt MX whose K_sp=[M⁺][K^-]

If this were dissolved in a solution of the acid HX whose K_a=[H⁺][X^-]/[HX]
then what would be its solubility?

Now K_a/[H⁺]=[X^-]/[HX]

or K_a/([H⁺]+ K_a)=[X^-]/([HX]+[X^-])=f say
It can be seen that f decreases as pH decreases.

If S were the solubility,

Then K_sp=(S)(f*S)
= S²(K_a+[H⁺])/K_a

Could anyone please explain how K_sp=(S)(f*S) because I have no idea