Question

The \(\mathrm{pH}\) of the neutralization point of \(0.1 \mathrm{~N}\) ammonium hydroxide with \(0.1 \mathrm{NHCl}\) is: (a) 1 (b) 6 (c) 9 (d) 7

Short answer

The pH at the neutralization point is 7.

Step-by-step solution

Analyze the chemical reaction

Ammonium hydroxide (\( ext{NH}_4 ext{OH} \)) is a weak base and hydrochloric acid (\( ext{HCl} \)) is a strong acid. When these two react, they form water (\( ext{H}_2 ext{O} \)) and ammonium chloride (\( ext{NH}_4 ext{Cl} \)) according to the equation:\[\text{NH}_4\text{OH} + \text{HCl} \rightarrow \text{NH}_4\text{Cl} + \text{H}_2\text{O}\]

Key concepts

Ammonium Hydroxide

Ammonium hydroxide, also known as \(\text{NH}_4\text{OH}\), is a solution of ammonia gas dissolved in water. It's an example of a weak base, which means it doesn't completely dissociate in water. As a weak base, ammonium hydroxide exists in equilibrium with its ionized form, ammonia (\(\text{NH}_3\)) and the hydroxide ion (\(\text{OH}^-\)).

It's essential to recognize what a weak base means in chemistry:

• It does not ionize fully in solution.
• Only a fraction of the ammonium hydroxide molecules dissociate to form hydroxide ions.
• It results in a lower concentration of \(\text{OH}^-\) ions compared to a strong base of the same concentration.

In the context of a neutralization reaction, ammonium hydroxide behaves differently than a strong base. Its contribution to the solution’s alkalinity is less intense, which impacts the \text{pH}\ after neutralization.

pH Calculation

Calculating \text{pH}\ in a reaction involving a weak base like ammonium hydroxide requires understanding how \text{pH}\ is impacted by the balance of acid and base. Typically, the \text{pH}\ scale ranges from 0 to 14, where values below 7 indicate an acidic solution, and values above 7 indicate a basic one. Neutral solutions are exactly at \text{pH}\ 7.

For ammonium hydroxide neutralized by hydrochloric acid in equal normality (0.1 \text{N}\ each), the resulting solution will not be neutral with a \text{pH}\ of 7 due to the nature of the weak base. Instead, the \text{pH}\ after complete neutralization tends towards being slightly acidic due to the weak base strong acid combination. Ammonium ion (\(\text{NH}_4^+\)) is a conjugate acid of ammonia, and when formed it slightly lowers the \text{pH}\ of the solution.

Weak Base Strong Acid Neutralization

The neutralization reaction between a weak base like ammonium hydroxide and a strong acid such as hydrochloric acid is an interesting chemical process. During this reaction, each molecule of \(\text{NH}_4\text{OH}\) combines with \(\text{HCl}\) to form \(\text{NH}_4\text{Cl}\) and water (\(\text{H}_2\text{O}\)).

Here’s what typically happens:

• The strong acid provides more \(\text{H}^+\) ions than needed to neutralize the hydroxide ions from the weak base, driving the formation of an acidic salt.
• As \(\text{NH}_4^+\) ions form, they shift the equilibrium, slightly lowering the \text{pH}\ of the solution.
• This results in a neutralization point \text{pH}\ below 7, usually around 5-6 depending on the exact conditions.

This phenomenon emphasizes that the reaction’s outcome doesn’t lead to a perfectly neutral \text{pH}\, due to the limited disassociation and weaker nature of the base.