Without doing any calculations, determine if pH=p Ka, pH>p Ka, or pH<p Ka . Assume that HA is a

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Without doing any calculations, determine if pH=p Ka, pH>p...

Without doing any calculations, determine if $\mathrm{pH}=\mathrm{p} K_{\mathrm{a}}, \mathrm{pH}>\mathrm{p} K_{\mathrm{a}},$ or $\mathrm{pH}<\mathrm{p} K_{\mathrm{a}} .$ Assume that $\mathrm{HA}$ is a weak monoprotic acid. a. 0.10 mol $\mathrm{HA}$ and 0.050 $\mathrm{mol}$ of $\mathrm{A}^{-}$ in 1.0 $\mathrm{L}$ of solution b. 0.10 $\mathrm{mol} \mathrm{HA}$ and 0.150 $\mathrm{mol}$ of $\mathrm{A}^{-}$ in 1.0 $\mathrm{L}$ of solution c. 0.10 mol $\mathrm{HA}$ and 0.050 $\mathrm{mol}$ of $\mathrm{OH}^{-}$ in 1.0 $\mathrm{L}$ of solution d. 0.10 $\mathrm{mol} \mathrm{HA}$ and 0.075 $\mathrm{mol}$ of $\mathrm{OH}^{-}$ in 1.0 $\mathrm{L}$ of solution

Without doing any calculations, determine if $\mathrm{pH}=\mathrm{p} K_{\mathrm{a}}, \mathrm{pH}>\mathrm{p} K_{\mathrm{a}},$ or $\mathrm{pH}<\mathrm{p} K_{\mathrm{a}} .$ Assume that $\mathrm{HA}$ is a weak monoprotic acid.
a. 0.10 mol $\mathrm{HA}$ and 0.050 $\mathrm{mol}$ of $\mathrm{A}^{-}$ in 1.0 $\mathrm{L}$ of solution
b. 0.10 $\mathrm{mol} \mathrm{HA}$ and 0.150 $\mathrm{mol}$ of $\mathrm{A}^{-}$ in 1.0 $\mathrm{L}$ of solution
c. 0.10 mol $\mathrm{HA}$ and 0.050 $\mathrm{mol}$ of $\mathrm{OH}^{-}$ in 1.0 $\mathrm{L}$ of solution
d. 0.10 $\mathrm{mol} \mathrm{HA}$ and 0.075 $\mathrm{mol}$ of $\mathrm{OH}^{-}$ in 1.0 $\mathrm{L}$ of solution

Key Concepts

Henderson–Hasselbalch Equation

This equation relates the pH of a solution containing a weak acid and its conjugate base to the acid’s pKa and the ratio of the concentrations of the conjugate base and the acid. It provides insight into how the balance of acid and conjugate base controls the pH, with pH equal to pKa when the concentrations are equal, higher pH when the conjugate base concentration is greater, and lower pH when the acid concentration is higher.

Acid–Base Buffer Systems

A buffer system consists of a weak acid and its conjugate base, and it resists changes in pH when small amounts of acid or base are added. The pH of a buffer can be understood by considering the ratio of the conjugate base to the acid, as described by the Henderson–Hasselbalch Equation, which makes it possible to predict whether the pH is above, below, or equal to the pKa.

Acid–Base Reaction Stoichiometry

In mixtures containing a weak acid and either its conjugate base or a strong base, the stoichiometry of the reaction dictates the amounts of acid and conjugate base present. When a strong base is added, it neutralizes some of the weak acid to form additional conjugate base, altering the ratio of [A–] to [HA] and thereby affecting the pH relative to the pKa.

Relationship Between pH, pKa, and Conjugate Base/Acid Ratio

The ratio of the conjugate base to the acid determines whether the solution’s pH is equal to, greater than, or less than the pKa. When the ratio is one, the logarithmic term in the Henderson–Hasselbalch Equation becomes zero, making pH equal to pKa; a greater ratio leads to a pH higher than pKa, while a lower ratio results in a pH lower than pKa.

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