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Chapter 16: Problem 12

Why does a solution of a weak base and its conjugate acid control pH better than a solution of the weak base alone?

Short Answer

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Question: Explain why a solution of a weak base and its conjugate acid is better at controlling pH changes compared to a solution containing only the weak base. Answer: A solution of a weak base and its conjugate acid is better at controlling pH changes because it acts as a buffer, neutralizing added acids or bases and minimizing pH changes. In a solution containing only the weak base, there is no buffering action, and pH changes are more significant when acids or bases are added.

Step by step solution

01

Understanding how a buffer system works

A buffer is a mixture of a weak acid or a weak base and its conjugate partner that resists significant changes in pH when an acid or a base is added. It works by neutralizing the added acid or base in the solution to maintain a nearly constant pH. The buffering action depends on a presence of relatively equal amounts of a weak base and its conjugate acid.
02

The role of a weak base in controlling pH

When a weak base (e.g., ammonia, NH3) is dissolved in water, it only partially ionizes by accepting a proton (H+) from water molecules, forming its conjugate acid (ammonium ion, NH4+) and a hydroxide ion (OH-). The equilibrium reaction is as follows: NH3 (aq) + H2O (l) <=> NH4+ (aq) + OH- (aq) This reaction does not proceed to a significant extent, leading to only a moderate increase in the concentration of OH- ions and, therefore, affecting the pH only slightly.
03

The role of a weak base and its conjugate acid in controlling pH

A solution containing a weak base and its conjugate acid, in relatively equal amounts, acts as a buffer. The buffering action allows the solution to better control pH changes. When an acid (H+) is added, the weak base (e.g., NH3) will react with the added H+ to form its conjugate acid (NH4+): NH3 (aq) + H+ (aq) => NH4+ (aq) When a base (OH-) is added, the conjugate acid (e.g., NH4+) will react with the added OH- to form the weak base (NH3) and water: NH4+ (aq) + OH- (aq) => NH3 (aq) + H2O (l) In both cases, the added acid or base is neutralized, and the pH changes are minimized due to the buffering action.
04

Comparing the solutions of weak base alone and weak base with conjugate acid in controlling pH

A solution of the weak base alone is less effective in controlling pH changes as it does not have the buffering action of a weak base/conjugate acid mixture. When acids or bases are added to a solution of weak base alone, the pH will change more significantly because there are no species present to neutralize the added H+ or OH- ions, unlike in the buffer system. Thus, a solution of a weak base and its conjugate acid is better at controlling pH compared to a solution containing only the weak base.

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