Which of the following will produce a solution with pH less than 7 at equivalent point?

Analysis

The question asks us to identify which combination of acid and base will result in a solution with a pH less than 7 at the equivalence point.

A pH less than 7 indicates an acidic solution. In acid-base titrations, the pH at the equivalence point depends on the strength of the acid and the base involved:

1. Strong Acid + Strong Base: Produces a neutral salt (pH = 7).
2. Strong Acid + Weak Base: Produces an acidic salt (pH < 7).
3. Weak Acid + Strong Base: Produces a basic salt (pH > 7).

Concept Explanation

To solve this, we must identify the strength of the reactants in each option:

• Strong Acids: $HCl$, $HNO_3$, $H_2SO_4$.
• Strong Bases: $NaOH$, $KOH$.
• Weak Bases: $Mg(OH)_2$ (Magnesium hydroxide is sparingly soluble and acts as a weak base).

When a strong acid reacts with a weak base, the resulting salt undergoes hydrolysis. The cation from the weak base reacts with water to release hydrogen ions ($H^+$), making the solution acidic.

Step-by-Step Solution

Let's evaluate the options:

• A. $HNO_3$ (Strong Acid) + $NaOH$ (Strong Base): These react to form a neutral salt ($NaNO_3$). The pH is 7.
• B. $H_2SO_4$ (Strong Acid) + $KOH$ (Strong Base): These react to form a neutral salt ($K_2SO_4$). The pH is 7.
• C. $HCl$ (Strong Acid) + $Mg(OH)_2$ (Weak Base): The reaction is: $2HCl + Mg(OH)_2 \rightarrow MgCl_2 + 2H_2O$ Because $Mg(OH)_2$ is a weak base, the $Mg^{2+}$ ions undergo hydrolysis in water to produce $H^+$ ions, resulting in a solution with a pH < 7.
• D. $HNO_3$ (Strong Acid) + $KOH$ (Strong Base): These react to form a neutral salt ($KNO_3$). The pH is 7.

Conclusion

Option C is the only combination involving a weak base, which leads to an acidic equivalence point.

Exam Tip

Always remember the "Stronger Wins" rule for salt hydrolysis:

• If the acid is stronger than the base, the salt is acidic (pH < 7).
• If the base is stronger than the acid, the salt is basic (pH > 7).
• If both are strong (or both are weak), the salt is neutral (pH = 7).