medium · MCAT chem-phys
A weak monoprotic acid HA (K_a = 1.8 × 10^-5) is titrated with strong base. A researcher prepares a buffer at exactly the half-equivalence point, then dilutes it 100-fold with pure water at constant temperature.
Which statement best describes the effect on the solution's pH?
- The pH stays essentially constant because the ratio [A^-]/[HA] is unchanged, while the slight rise reflects HA's increased fractional dissociation on dilution
- The pH falls by about 2 units because diluting the buffer 100-fold overwhelms the H+ buffering capacity, letting the concentration term dominate the log expression
- The pH rises by exactly 2 units because each 10-fold dilution of any buffer is assumed to shift pH by a full unit toward neutrality, no matter what the conjugate acid-base ratio is
- The pH is undefined at the half-equivalence point because HA and A- occur in equal concentration, which the student assumes makes the Henderson-Hasselbalch ratio mathematically indeterminate
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