Question

What will be pH of aqueous salt solution of AB if pKa Of weak acid HA is $ 4 $ and pKb of weak acid BOH is $ 5 $ ?
(A) $ 5 $
(B) $ 7.5 $
(C) $ 6.5 $
(D) $ 11.5 $

Answer 1

Hint :When acids and bases react with each other, salts and water molecules are formed. Salts can be acidic, basic or neutral in nature according to the nature of the acid and base involved in the reaction. An acidic salt will have pH less than $ 7 $ and a basic salt will have pH greater than $ 7 $ . The neutral salt will have the pH equals to $ 7 $ . We can find the pH of salt formed by weak acid and base with the help of pKa of acid and pKb of base.

Complete Step By Step Answer:
When acids and bases react with each other, salts and water molecules are formed.
Strong acid and strong base react, they form a neutral salt whose pH is $ 7 $ .
Strong acid and weak base when react together forms an acidic salt with pH less than $ 7 $ .
Strong base and weak acid when react together forms a basic salt with pH greater than $ 7 $ .
Weak acid and weak base when react together forms a salt whose pH depends upon the dissociation constants of acid and base.
The pH can be calculated using formula:
 $ pH = \dfrac{1}{2}[p{K_w} + p{K_a} - p{K_b}] $
Now, given values are:
 $ p{K_w} = 14 $
 $ p{K_a} = 4 $
 $ p{K_b} = 5 $
Put these values in the above equation:
 $ pH = \dfrac{1}{2}[14 + 4 - 5] $
On solving this, we get
 $ pH = 6.5 $
Therefore, pH of aqueous salt solution of AB is found to be $ 6.5 $ .
Hence, the correct option is (C) $ 6.5 $ .

Note :
We should note that in all the above cases of acids bases that is strong acid-strong base, strong acid- weak base, weak acid-strong base and weak acid-weak base, the pH value of the aqueous solution of salt formed is always close to the neutral value that is $ 7 $ . This value cannot differ much from the neutral value.