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?

A buffered solution is a solution that resists significant changes in pH when a small amount of acid or base is added. It usually contains a weak acid and its conjugate base, or a weak base and its conjugate acid.

When acetic acid (CH3COOH) and sodium hydroxide (NaOH) are mixed, they undergo a neutralization reaction to form water and sodium acetate (CH3COONa). The reaction can be written as follows: \[ CH3COOH + NaOH \rightarrow CH3COONa + H2O \]

The neutralization reaction between acetic acid and sodium hydroxide forms sodium acetate, which can dissociate into acetate ions (CH3COO-) and sodium ions (Na+). The presence of acetic acid and acetate ions creates a buffer system, as acetic acid is a weak acid and acetate ions act as its conjugate base. In this buffer system, acetic acid can donate a proton (H+) to the solution and form acetate ions, while acetate ions can capture a proton from the solution to form acetic acid. This proton-exchange equilibrium between acetic acid and acetate ions is the basis of the buffer's effectiveness.

The effectiveness of the buffer depends on the ratio of the concentration of the weak acid (acetic acid) to the concentration of its conjugate base (acetate ions). This ratio influences the pH of the buffered solution, as described by the Henderson-Hasselbalch equation: \[ pH = pK_a + \log{\frac{[conjugate\: base]}{[weak\: acid]}}\] By changing the amounts of acetic acid and sodium hydroxide added to the solution, we can alter the concentration ratio of acetic acid to acetate ions. However, adding too much sodium hydroxide can completely neutralize the acetic acid, at which point there will no longer be a buffering effect. Similarly, if not enough sodium hydroxide is added, the buffer's concentration of acetate ions will be low, reducing its effectiveness. Therefore, the effectiveness of the buffer depends on achieving an appropriate ratio of acetic acid and acetate ions—neither too high nor too low.