In the previous post couple of posts, we learned how to determine whether the solution of a salt is going to be acidic or basic, and calculate the pH of a salt solution.

In short, if the salt is made of strong acid and a weak base, then it will give an acidic solution. If it is prepared from a strong base and weak aid, the solution will be basic, and if it is a combination of a strong acid and a strong base, a neutral solution will be formed.

 

 

Now, what if the salt contains a cation of a strong base and an anion of a strong acid. With this combination, the salt contains an acidic cation and a basic anion, and the question is whether the solution is going to be acidic or basic.

 

When K_a < K_b, a Basic Solution is Formed

For example, ammonium carbonate (NH₄)₂CO₃ contains a cation of a weak base ammonia (NH₃) and an anion of a weak acid H₂CO₃.

Remember, when determining the pH of a salt solution, we identified the cation of the weak component and reacted it with water to see if it produces H₃O⁺ or OH^– ions. And now, because both ions are from a weak acid and base, we need to react them with water and see which one produces more H₃O⁺ or OH^–.

 

NH₄⁺(aq) + H₂O(l) → NH₃(aq) + H₃O⁺(aq) (makes acidic)

CO₃^2-(aq) + H₂O(l) → HCO₃^–(aq) + OH^–(aq) (makes basic)

 

If the hydrolysis of NH₄⁺ produces more H₃O⁺ than CO₃^2- produces OH^–, the solution will be acidic. And vice versa, if there is more OH- produced in the solution, it will become a basic solution.

So, how do we know which of the ions will be produced in a greater quantity?

This is determined based on the values of K_a and K_b.

 

NH₄⁺(aq) + H₂O(l) → NH₃(aq) + H₃O⁺(aq) K_a = 5.6 x 10^-10

CO₃^2-(aq) + H₂O(l) → HCO₃^–(aq) + OH^–(aq) K_b = 1.8 x 10^-4

 

The K_b of carbonate ion is larger than the Ka of ammonia, and therefore, the solution will be basic because there is more OH- than H₃O⁺ produced in the solution.

 

When K_a > K_b, an Acidic Solution is Formed

Let’s also consider an example of a salt that yields an acidic solution.

For example, The NH₄F is salt of a weak base ammonia and a weak acid HF. To determine whether the solution is going to be basic or acidic, we write the reaction of the cation and the anion with water:

NH₄⁺(aq) + H₂O(l) → NH₃(aq) + H₃O⁺(aq) K_a = 5.6 x 10^-10

F^–(aq) + H₂O(l) → HF(aq) + OH^–(aq) K_b = 2.9 x 10^-11

 

Comparing the values of K_a for the acidic cation and K_b for the basic anion, we determine the solution will be acidic because K_a > K_b.

 

When K_a ≈ K_b, a Neutral Solution is Formed

One more possibility is when the salt of a weak acid and weak base produces a neutral solution. This is somewhat similar to what we saw for salts containing strong acid and base components. However, the difference is that in this case, there is a reaction with water, but because K_a ≈ K_b, the hydronium and hydroxide ions are formed in approximately equal quantities and the solution stays neutral.

 

In summary, remember these patterns of the K_a and K_b relationship when determining the acidity of salt solution.

 

• K_a > K_b: If K_a for the cation is greater than Kb for the anion, the first reaction will produce more H₃O⁺ and therefore, pH < 7.
• K_a < K_b: If K_a for the cation is less than Kb for the anion, the second reaction will produce more OH^– and therefore, pH > 7.
• K_a ≈ K_b: If K_a for the cation and K_b for the anion are about equal, the concentrations of H₃O⁺ and OH^– ions will be balanced, so the pH ≈ 0.

 

Check Also

• Definitions of Acids and Bases
• Acid-Base Reactions
• Acid-Base Titrations
• Conjugate Acid and Conjugate Base
• Autoionization of Water and K_w
• The pH and Acidity
• Acid Strength, K_a, and pK_a
• Base Strength, K_b and pK_b
• K_a, pK_a, K_b, and pK_b Relationship
• The pH of a Strong Acid and Base
• pH + pOH = 14
• The pH of a Weak Acid
• The pH of a Weak Base
• ThepH of Polyprotic Acids
• The acidity of a Salt Solution
• The pH of a Salt Solution
• Acids and Bases Practice Problems