Buffer action check: When 1 cm^3 of dilute hydrochloric acid (1% by volume) is added to 80 cm^3 of a buffer solution initially at pH = 4, what will be the resulting pH (qualitatively)?

Introduction / Context:
Buffers resist changes in pH when small amounts of acid or base are added. This robustness is central in biochemical assays, fermentations, and analytical chemistry where stable hydrogen ion activity is critical.

Given Data / Assumptions:

• Buffer volume: 80 cm^3 at pH 4.
• Added acid: 1 cm^3 of 1% HCl (very small, dilute addition).
• Assume buffer capacity is adequate; temperature effects negligible.

Concept / Approach:
The Henderson–Hasselbalch relation shows that pH depends on the ratio base/acid forms. Small additions cause minimal ratio changes if the buffer capacity is not exhausted. A tiny acid addition to a relatively large buffer volume will not significantly shift pH.

Step-by-Step Solution:
Estimate moles of added H^+ (very small due to 1% and tiny volume).Compare to moles of buffer components (much larger).Conclude the ratio changes insignificantly; pH remains approximately 4.Hence, select pH = 4.

Verification / Alternative check:
Quantitative buffer capacity calculations (β) confirm millimole-level buffering can neutralize micromole additions with negligible pH change.

Why Other Options Are Wrong:
pH 1 or 2 implies overwhelming acid addition; pH 8 implies base addition, contrary to the scenario.

Common Pitfalls:

• Forgetting that buffer capacity is finite; here, however, the addition is very small.
• Assuming any acid addition drastically changes pH regardless of buffer volume.

Final Answer:
pH = 4