Question

Which buffer resists changes in pH most effectively?

• A. \(\mathrm{pH}=2,\; \mathrm{p}K_a=2\)
• B. \(\mathrm{pH}=4,\; \mathrm{p}K_a=5\)
• C. \(\mathrm{pH}=7,\; \mathrm{p}K_a=3\)
• D. \(\mathrm{pH}=9,\; \mathrm{p}K_a=5\)

Hint:

A buffer is most effective when \[ \boxed{\mathrm{pH}=\mathrm{p}K_a.} \] The buffering range is approximately \[ \boxed{\mathrm{p}K_a\pm1.} \]

Answer 1

The Correct Option is A

Step 1: Recall the condition for maximum buffer capacity. According to the Henderson--Hasselbalch equation, \[ \boxed{ \mathrm{pH}=\mathrm{p}K_a+\log\frac{[\mathrm{Salt}]}{[\mathrm{Acid}]} } \] A buffer shows maximum resistance to change in pH when \[ \boxed{\mathrm{pH}=\mathrm{p}K_a.} \]

Step 2: Check the given options. Among the given options, \[ \mathrm{pH}=2,\qquad \mathrm{p}K_a=2 \] satisfies \[ \boxed{\mathrm{pH}=\mathrm{p}K_a.} \] Hence, this buffer has the maximum buffering capacity. Therefore, \[ \boxed{(A)} \] is the correct answer.