Higher the amount of acid or base used to product a deinite change of pH in a buffer solution, higher will be its buffe capacity. Buffer capcity of solution is maximum under the following conditions
[Salt] = [Acid](in acid buffer)
[Salt] = [Base] (in base capacity]
pH of buffer solution lies inthe range given below `pH = pH_(a) +-1`
In other words,any buffer solution can be used as buffer up to two pH units only, depending upon the value of `pK_(a)` or `pK_(b)`. A buffer is said to be efficient when `pH = pK_(a)` or `pOH = pK_(b)`
`K_(a)` for `CH_(3)COOH` is `1.8 xx 10^(-5)`and `K_(b)` for `NH_(4)OH` is `1.8 xx 10^(-5)`. The ph of ammonium acetate will be

Options

A. `7.005`
B. `4.75`
C. `7.0`
D. Between 6 and 7

Answer

C

Explanation

`K_(a)` for `CH_(3)COOH = 1.8 xx 10^(-5)`
`K_(b)` for `NH_(4)OH = 1.8 xx 10^(-5)`
Ammonium acetate is a salt of weak acid and weak base. For such salts
`pH = 7 + (pK_(a)-pK_(b))/2 `
`rArr 7+ ([-log 1.8 xx 10^(-5)]-[-log 1.8 xx 10^(-5)])/2 `
`rArr 7 + (4.74 -4.74)/2 = 7.00`