Question

In which of the following condition, a mixture has the ability to act as an acidic buffer?

• A. $\frac{\left[C{H}_{3}CO{O}^{-}\right]}{\left[C{H}_{3}COOH\right]}<0.1$
• B. $0.1<\frac{\left[C{H}_{3}CO{O}^{-}\right]}{\left[C{H}_{3}COOH\right]}<10$
• C. $\frac{\left[C{H}_{3}CO{O}^{-}\right]}{\left[C{H}_{3}COOH\right]}>10$
• D. None of the above

Answer 1

The correct option is B $0.1<\frac{\left[C{H}_{3}CO{O}^{-}\right]}{\left[C{H}_{3}COOH\right]}<10$

Buffer range :
If $\frac{\left[C{H}_{3}CO{O}^{-}\right]}{\left[C{H}_{3}COOH\right]}<0.1$ then it can be treated as weak acid only.
$pH=\frac{1}{2}(p{K}_a-logC)$

If $0.1<\frac{\left[C{H}_{3}CO{O}^{-}\right]}{\left[C{H}_{3}COOH\right]}<10$, then both the concentrations with respect to each other cannot be neglected. Thus, it can act as an acidic buffer.
$pH=p{K}_a+\frac{\left[C{H}_{3}CO{O}^{-}\right]}{\left[C{H}_{3}COOH\right]}$

If $\frac{\left[C{H}_{3}CO{O}^{-}\right]}{\left[C{H}_{3}COOH\right]}>10$ then salt hydrolysis can be assumed.
$pH=\frac{1}{2}(p{K}_a+p{K}_w+logC)$
For example, those acetic acid and sodium acetate mixture can be used as buffer solutions whose pH values are roughly in the range of 3.74 to 5.74.
Outside this range, the buffer capacity of acetic acid–sodium acetate solution is too small to be used for any practical purpose.