Question

0.1N solution of sodium acetate will have Ph _ _ _ _ _ _ ? ($p{K_a}$ = 4.57)
 (A) 8.78
(B) 11.57
(C) 4.57
(D) 7.0

Answer 1

Hint: First check which type of salt is sodium acetate, which means it is made up of which type of acid and base. Then accordingly use the formula for calculating its pH.

Complete step by step answer:
- Sodium acetate has a molecular formula of $C{H_3}COONa$. It is a sodium salt of acetic acid. It is a colourless deliquescent salt. It is a basic salt since the acetate ion is capable of deprotonation of water and ultimately raising the solution’s pH.
- It has some harmful effects as well. Its inhalation causes cough and sore throat, and direct skin or eye contact may also cause redness, irritation and itching. But overall toxicity of sodium acetate is minimal.
It can be produced by the reaction of acetic acid with sodium carbonate, sodium bicarbonate or sodium hydroxide. The reaction can be written as:
$C{H_3}COOH + NaHC{O_3} \to C{H_3}COONa + {H_2}C{O_3}$
Hence we can also say that sodium acetate is a salt of weak acid (acetic acid) and a strong base (sodium hydroxide).
- We should also know what pH is. The measure of any substance about how acidic or basic it is can be called as pH. It ranges from 0 to 14.
- For this kind of salt pH can be written as:
$pH = 7 + \frac{1}{2}p{K_a} + \frac{1}{2}\log C$ (1)
Where the values given in the question are:
C = concentration of salt = 0.1N
$p{K_a}$ = 4.57
Putting these values in equation (1):
$pH = 7 + \frac{1}{2}(4.57) + \frac{1}{2}\log (0.1)$
(log 0.1 = -1)
pH = 7 + 2.285 + (0.5)(-1)
   = 8.785
So, the correct option will be: (A) 8.78.

Note: Sodium bicarbonate is an amphoteric compound and its aqueous solutions are mildly alkaline due to formation of carbonic acid and hydroxide ion. So, when it reacts with an acid it produces salt and carbonic acid, which further decomposes into carbon dioxide and water.