You dissolve 0.425 g of NaOH in 2.00 L of a buffer solution that has [H2 PO4 ^-]=[HPO4 ^2-]=0.

step 1 If we start with a buffer solution that contains equal concentrations of dihydrogen phosphate an

You dissolve 0.425 g of NaOH in 2.00 L of a buffer...

You dissolve $0.425 \mathrm{~g}$ of $\mathrm{NaOH}$ in $2.00 \mathrm{~L}$ of a buffer solution that has $\left[\mathrm{H}_2 \mathrm{PO}_4{ }^{-}\right]=\left[\mathrm{HPO}_4{ }^{2-}\right]=0.132 \mathrm{M}$. What is the $\mathrm{pH}$ of the solution before adding $\mathrm{NaOH}$ ? After adding $\mathrm{NaOH}$ ?

Key Concepts

Buffer Solution

A buffer solution is comprised of a weak acid and its conjugate base (or a weak base and its conjugate acid) in comparable amounts, which acts to resist changes in pH when small amounts of a strong acid or base are introduced. Its ability to maintain a nearly constant pH stems from the equilibrium between the acid and its conjugate base that neutralizes added H+ or OH? ions.

Acid-Base Equilibrium

Acid-base equilibrium refers to the reversible reactions involving proton transfer between acids and bases in solution. In a buffer, this equilibrium ensures that added acids or bases are counteracted by shifts in the reaction, limiting drastic changes in the concentration of H+ ions, which in turn helps stabilize the pH.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is a simplified form of the acid-base equilibrium expression that relates the pH of a solution to the pKa of the weak acid and the ratio of the concentrations of its conjugate base and acid. This equation is especially useful in calculating the pH of buffer solutions and understanding the effect of adding small amounts of acid or base on the system.

Neutralization Reaction

In the context of a buffer system, a neutralization reaction occurs when a strong acid or base reacts with one component of the buffer, converting it into its conjugate form. For example, a strong base like NaOH will react with the weak acid component of the buffer, reducing its concentration and increasing that of the conjugate base, thereby altering the pH according to the buffer's capacity.

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