Solving for pH of a Salt Solution

Category: Chemical Engineering Math

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What is the pH of 0.256 M NH₄Cl? K_b = 1.8 x 10^-5 for NH₄OH.

Solution:

The given salt, which is NH₄Cl is an acidic salt because it is a product of weak base and strong acid and HCl is a strong acid. In this case, the pH of an acidic salt is less than 7. The pH of a neutral salt is always equal to 7. Neutral salt is a product of strong acid and strong base or a product of weak acid and weak base. 

Let's consider the ionization of NH₄Cl as follows
 

 

 

Since [H⁺] is a product during the ionization process, then we have to get K_a from K_b as follows

The K_a of the ionization process of ammonium ion is 

If x is the concentration of [H⁺], then it follows that
 

If the concentration of NH₄Cl is 0.256 M, then it follows that
 

Hence, the concentration of [H⁺] during the ionization process is
 

 

Since the value of K_a is very small, then we can neglect x at the denominator as follows
 

 

 

 

 

Therefore, the pH of 0.256 M NH₄Cl is