Calculate the pH of a solution that is 0.20 M HOCl and 0.90 M KOCl. In order for this buffer to

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Calculate the pH of a solution that is 0.20 M HOCl and 0.90...

Calculate the $\mathrm{pH}$ of a solution that is $0.20 \mathrm{M} \mathrm{HOCl}$ and $0.90 \mathrm{M}$ KOCl. In order for this buffer to have $\mathrm{pH}=\mathrm{p} K_{\mathrm{a}}$, would you add HCl or NaOH? What quantity (moles) of which reagent would you add to $1.0 \mathrm{~L}$ of the original buffer so that the resulting solution has $\mathrm{pH}=\mathrm{p} K_{\mathrm{a}}$ ?

Key Concepts

Buffer Solutions

A buffer solution is a system that resists changes in pH when small amounts of acid or base are added. It consists typically of a weak acid and its conjugate base (or a weak base and its conjugate acid), allowing the solution to neutralize added H+ or OH- ions through equilibrium reactions. This concept is key to understanding how a mixture can maintain a relatively constant pH despite disturbances.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates the pH of a buffer solution to the pKa of the weak acid and the ratio of the concentrations of the conjugate base to the acid. It is used to determine the pH of a buffer or to design a buffer with a target pH by adjusting the ratio of acid to conjugate base. This equation is vital for predicting and controlling the pH in acid-base systems.

pKa

The pKa is the negative logarithm of the acid dissociation constant (Ka) for a weak acid and provides a measure of its strength. It represents the pH at which half of the acid is dissociated, meaning the concentrations of the acid and its conjugate base are equal. Knowing the pKa is crucial for buffer preparation because when pH equals pKa, the buffering capacity is maximized.

Acid-Base Neutralization in Buffers

In buffer systems, adding a strong acid or base triggers neutralization reactions that shift the balance between the weak acid and its conjugate base. The addition of a strong acid converts some of the conjugate base to the weak acid, while adding a strong base does the reverse. This stoichiometric adjustment is essential for fine-tuning the pH of the solution towards the desired value, particularly when adjusting a buffer to achieve a specific pH related to its pKa.

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