Question

Mixing together solutions of acetic acid and sodium hydroxide can make a buffered solution. Explain. How does the amount of each solution added change the effectiveness of the buffer?

Short answer

Mixing acetic acid and sodium hydroxide forms a buffered solution consisting of acetic acid (weak acid) and sodium acetate (conjugate base), which help maintain a near constant pH. The balanced equation for the reaction is: \(CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O\). The buffer capacity depends on the concentrations of the weak acid and its conjugate base. Adjusting the amounts of acetic acid and sodium hydroxide solutions affects the concentration ratio, which in turn influences the buffer's effectiveness in maintaining a constant pH.

Step-by-step solution

1. Write the reaction equation

Write the balanced equation for the reaction between acetic acid and sodium hydroxide, forming sodium acetate and water. \[CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O\]

2. Understand the buffer system

In the reaction, acetic acid (weak acid) and sodium acetate (conjugate base) form a buffer system. This is because they can both donate or accept protons (H+) depending on whether an acid or base is added to the solution, helping to maintain the pH levels constant. For example, if an acid is added to the buffer system, the conjugate base \(CH_3COO^-\) will accept protons: \[CH_3COO^- + H^+ \rightarrow CH_3COOH\] If a base is added to the buffer system, the acetic acid will donate protons: \[CH_3COOH + OH^- \rightarrow CH_3COO^- + H_2O\]

3. Determine the buffer capacity

The buffer capacity is defined as the amount of acid or base that can be added to a buffered solution before its pH changes significantly. The buffer capacity depends on the concentrations of the weak acid and its conjugate base. A higher concentration of both the weak acid and its conjugate base will result in a higher buffer capacity, meaning it can resist pH changes more effectively.

4. Explore the effect of changing the amount of each solution

Changing the amount of acetic acid or sodium hydroxide in the solution will affect the buffer’s concentration ratio and therefore its effectiveness. If more acetic acid is added, the solution will become more acidic, reducing its ability to resist pH changes caused by adding more acid. If more sodium hydroxide is added, the solution will become more basic, making it less effective in resisting pH changes caused by adding a base. A balance between the concentration of acetic acid and sodium acetate is necessary for an effective buffer system that is able to resist pH changes.

5. Conclusion

Mixing solutions of acetic acid and sodium hydroxide can produce a buffered solution, with acetic acid and sodium acetate acting as the weak acid and conjugate base, respectively. Their concentrations determine the buffer capacity, and thus the effectiveness of the buffer in maintaining a constant pH. Adjusting the amounts of acetic acid and sodium hydroxide solutions will affect the concentration ratio of the weak acid and its conjugate base, which in turn influences the buffer's effectiveness.