Titra9on of a Weak Acid with a Strong Base??

Question

Titrate 50 mL of 0.200 M HCN with 0.200 M KOH and determine the pH of the mixture at the following points of the titration curve:

a. When 0.0 mL of KOH has been added

b. When 25.0 mL of KOH has been added

c. When 50.0 mL of KOH has been added

d. When 75.0 mL of KOH has been added 

Do I subtract or add the excess moles?

Answer 1

(a)

Step 1:

The amount of HCN is 50 ml = 0.05 l.

Molarity of the HCN is 0.2 M.

Equilibrium constant for HCN is .

Calculate pH when 0.0 mL of KOH has been added.

HCN is at equilibrium.

The formula for equilibrium constant .

At equilibrium .

pH definition:

It is the measure of the acidity and alkalinity of a solution.

pH is the negative log of the activity of the hydrogen ion.

Solution:

pH when 0.0 mL of KOH has been added is 4.95.

Answer 2

(b)

Step 1:

The amount of HCN is 50 ml = 0.05 l.

Molarity of the HCN is 0.2 M.

The amount of KOH added is 25 ml.

Molarity of KOH is 0.2 M.

Calculate pH when 25 ml of KOH has been added.

Moles of = Molarity * Volume

Moles of = 0.2 * 0.05

Moles of = 0.01

Moles of = 0.2 * 0.025

Moles of = 0.005

moles acid in excess = .

Total volume = 0.05 + 0.025 = 0.075

pH definition:

It is the measure of the acidity and alkalinity of a solution.

pH is the negative log of the activity of the hydrogen ion.

Solution:

pH when 25 mL of KOH has been added is 1.17.