Your job is to determine the concentration of ammonia in a commercial window cleaner. In the titration of a 25.0 mL sample of the cleaner, the equivalence point is reached after 23.8 mL of 0.164 M HCl has been added. What is the initial concentration of ammonia in the solution? What is the pH of the solution at the equivalence point?

The initial concentration of ammonia is 0.14 M and the pH of the solution at equivalence point is 5.20

Explanation:

To calculate the number of moles for given molarity, we use the equation:

     .....(1)

Molarity of HCl solution = 0.164 M

Volume of solution = 23.8 mL = 0.0238 L    (Conversion factor:  1 L = 1000 mL)

Putting values in equation 1, we get:

The chemical equation for the reaction of ammonia and HCl follows:

By Stoichiometry of the reaction:

1 mole of HCl reacts with 1 mole of ammonia

So, 0.0035 moles of HCl will react with = of ammonia

Moles of ammonia = 0.0035 moles

Volume of solution = 25 mL = 0.025 L

Putting values in equation 1, we get:

By Stoichiometry of the reaction:

1 mole of ammonia produces 1 mole of ammonium ion

So, 0.0035 moles of ammonia will react with = of ammonium ion

Moles of ammonium ion = 0.0035 moles

Volume of solution = [23.8 + 25] mL = 48.8 mL = 0.0488 L

Putting values in equation 1, we get:

where,

= Ionic product of water =

= Acid dissociation constant

= Base dissociation constant =

The chemical equation for the dissociation of ammonium ion follows:

The expression of for above equation follows:

We know that:

Putting values in above expression, we get:

To calculate the pH concentration, we use the equation:

We are given:

Hence, the initial concentration of ammonia is 0.14 M and the pH of the solution at equivalence point is 5.20