A buffer contains a weak acid, HX, and its conjugate base....

A buffer contains a weak acid, HX, and its conjugate base. The weak acid has a $\mathrm{p} K_{a}$ of $4.5,$ and the buffer has a $\mathrm{pH}$ of $4.3 .$ Without doing a calculation, predict whether $[\mathrm{HX}]=\left[\mathrm{X}^{-}\right]$ $[\mathrm{HX}]>\left[\mathrm{X}^{-}\right],$ or $[\mathrm{HX}]<\left[\mathrm{X}^{-}\right] .$ Explain. $[$ Section 17.2$]$

Key Concepts

Buffer Solution

A buffer solution is composed of a weak acid and its conjugate base (or a weak base and its conjugate acid) and is designed to resist changes in pH when small amounts of acid or base are added. Buffers maintain pH stability, which is essential in many chemical, biological, and industrial applications.

Henderson-Hasselbalch Equation

This equation, written as pH = pKa + log([base]/[acid]), establishes the relationship between the pH of a buffer solution, the acid dissociation constant (pKa) of the weak acid, and the ratio of the concentrations of the conjugate base to the weak acid. It is a key tool for understanding and designing buffer systems.

Relationship Between pH and pKa

The relationship between pH and pKa determines the composition of a buffer solution. If the pH is below the pKa, the equilibrium of the weak acid favors the protonated form (the weak acid), and if the pH is above the pKa, the deprotonated form (the conjugate base) is favored. This concept is crucial when predicting the qualitative ratio between a weak acid and its conjugate base in a buffer.

Weak Acid Equilibrium

Weak acids partially dissociate in solution, establishing an equilibrium between the undissociated acid and its ions. The position of this equilibrium is characterized by the acid's dissociation constant (Ka) or its logarithmic form (pKa). This equilibrium is fundamental in understanding how buffers work and how they maintain a stable pH by shifting between the acid and conjugate base forms.

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