Explain what happens to the concentration of the weak acid and the weak base in a buffer as a small amount of strong acid is added.
Question: Explain what happens to the concentration of the weak acid and the weak base in a buffer as a small amount of strong acid is added.
A buffer is a solution that resists changes in pH when a small amount of acid or base is added. A buffer consists of a weak acid and its conjugate base, or a weak base and its conjugate acid¹. When a small amount of strong acid is added to a buffer, the following happens:
- The strong acid reacts with the weak base in the buffer, reducing its concentration and producing more weak acid.
- The equilibrium of the weak acid shifts to the left, consuming some of the added acid and releasing some of the conjugate base.
- The pH of the buffer decreases slightly, but not as much as it would without the buffer.
For example, consider a buffer made of acetic acid (CH3COOH) and sodium acetate (CH3COONa). When a small amount of hydrochloric acid (HCl) is added, the following reactions occur:
- HCl + CH3COONa -> CH3COOH + NaCl
- CH3COOH + H2O <-> H3O+ + CH3COO-
The net effect is that the concentration of CH3COOH increases, the concentration of CH3COONa decreases, and the concentration of H3O+ increases slightly. The pH of the buffer decreases by a small amount, depending on the amount of acid added and the initial concentrations of the buffer components.
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