Question

What will be the ph of 0.1m ammonium formate solution if ka is 1*10^-4 acid kb is 1*10^-5?

Answer 1

Answer: pH of the 0.1M of above solution will be 6.5

Explanation: Ammonium formate is a salt of formic acid (weak acid) and ammonium hydroxide (weak base).

Calculation of pH can be done by using salt hydrolysis formula:

$pH=\frac{1}{2}(pK_w+pK_a-pK_b)$              ......(1)

Values of $K_a\text{ and }K_b$ are given:

$K_a=1\times 10^{-4}$

$K_b=1\times 10^{-5}$

To convert $K_a$ into $pK_a$, we use the relation:

$pK_a=-logK_a$

$pK_a=-log(0.0004)\\pK_a=4$

Similarly, we can calculate $pK_b$

$pK_b=5$

and $pK_w=14$    (known)

Putting above values in equation 1, we get

$pH=\frac{1}{2}(14+4-5)$

pH = 6.5