Question

Use the Henderson-Hasselbalch equation to calculate the pH of each solution:

a) a solution that is 0.170M in HC2H3O2 and 0.125M in KC2H3O2

Express your answer using two decimal places.

b) a solution that is 0.200M in CH3NH2 and 0.125M in CH3NH3Br

Express your answer using two decimal places.

Answer 1

(a) Here the acis is CH3COOH and the salt is CH3COOK

Given, [CH3COH] = 0.170M and [CH3COOK] = 0.125M

This is an example of acidic buffer and according to Henderson-Hasselbalch equation P^H  of an acidic buffer solution can be calculated as

P^H = P^Ka  + log[salt]/[acid] = P^Ka  + log[CH3COOK/[CH3COOH] =

Since for the weak acid CH3COOH, P^Ka = 4.76

P^H = 4.76 + log0.125/0.170 = 4.76+ (-0.134) = 4.63 (upto 2 decimal places) (answer)

(b) Here the base is CH3NH2 and the salt is CH3NH3Br

Given, [CH3NH2] = 0.200M and [CH3NH3Br] = 0.125M

This is an example of basic buffer and according to Henderson-Hasselbalch equation P^OH of a basic buffer solution can be calculated as

P^OH = P^Kb + log[salt]/[base] = P^Kb + log[CH3NH3Br]/[CH3NH2]

Since for the weak base CH3NH2, P^Kb= 3.36

P^OH = 3.36 + log0.125/0.200 = 3.16

Now P^H can be calculated as P^H = 14 - P^OH = 14 - 3.16 = 10.84 (upto 2 deimal places) (answer)