Solution of an acid and it's anion (that is it's conjugate base) or of a base and it's common cation are buffered. When we add a small amount of acid or base to any one of them, the pH of solution changes very little pH of buffer solution can be compoted
as , for , acidic , buffer
`pH = pK_(a) + "log"(["Conjugate base"])/(["Acid"])`
for basuc buffer
`pOH = pK_(b) + "log" (["Conjugated base"])/(["base"])`
it is generally accepteed that a solution has useful buffer capacity (`pH` change resistng power) provided that the value of [salt or conjugate base]/[acid] for acidic acid buffer lies within the range of `1:10` to `10:1`. Buffer capacity is max.
When [conjugate base]=[acid]
Calculate the pH of the solution made by adding `0.01` mole of `HCl` in `100` ml. of solution which is `0.2 M` in `NH_(3)(pK_(b) = 4.74)` and `0.3 M` in `NH_(4)^(+)`:

Options

A. `5.34`
B. `8.66`
C. `7.46`
D. `4.46`

Answer

B