Which statement about buffer solutions is true?

Which statement about buffer solutions is true?

• A. A buffer solution resists changes in pH upon addition of small amounts of acid or base.
• B. A buffer solution contains only strong acids.
• C. A buffer solution has a pH exactly 7.
• D. A buffer solution reacts completely with water.

Answer: (A)

A buffer resists changes in pH when small amounts of acid or base are added. This works because a buffer contains a weak acid and its conjugate base (or a weak base and its conjugate acid). If you add acid, the conjugate base neutralizes some of the added H⁺ by forming more of the weak acid; if you add base, the weak acid neutralizes the OH⁻ to form water and more of the conjugate base. This keeps the hydrogen ion concentration from shifting much. The pH of a buffer is linked to the acid’s strength (its pKa) and the ratio of conjugate base to acid, given by pH ≈ pKa + log([A⁻]/[HA]). When both components are present in appreciable amounts, the pH remains relatively stable across small additions of acid or base. The exact pH centers around the acid’s pKa, not necessarily at 7. For example, a solution of acetic acid and acetate has a pH near 4.76 when in balanced amounts; adjusting the ratio shifts the pH up or down within a range around that value. Buffer capacity increases with higher concentrations of the buffer components. The other statements don’t fit because a buffer relies on a weak acid with its conjugate base (not only strong acids), its pH is not fixed at 7, and it does not react completely with water—the buffer components remain in equilibrium, providing the buffering action.

A buffer resists changes in pH when small amounts of acid or base are added. This works because a buffer contains a weak acid and its conjugate base (or a weak base and its conjugate acid). If you add acid, the conjugate base neutralizes some of the added H⁺ by forming more of the weak acid; if you add base, the weak acid neutralizes the OH⁻ to form water and more of the conjugate base. This keeps the hydrogen ion concentration from shifting much.

The pH of a buffer is linked to the acid’s strength (its pKa) and the ratio of conjugate base to acid, given by pH ≈ pKa + log([A⁻]/[HA]). When both components are present in appreciable amounts, the pH remains relatively stable across small additions of acid or base. The exact pH centers around the acid’s pKa, not necessarily at 7.

For example, a solution of acetic acid and acetate has a pH near 4.76 when in balanced amounts; adjusting the ratio shifts the pH up or down within a range around that value. Buffer capacity increases with higher concentrations of the buffer components.

The other statements don’t fit because a buffer relies on a weak acid with its conjugate base (not only strong acids), its pH is not fixed at 7, and it does not react completely with water—the buffer components remain in equilibrium, providing the buffering action.