Question

An 80.0 mL sample of 0.200 M ammonia is titrated with 0.100M hydrochloric acid. Kb for ammonia is 1.8 x 10-5. Calculate the pH of the solution at each of the following points of the titration: a. before the addition of any HCl. ______________ b. halfway to the equivalence point.______________ c. at the equivalence point._________________ d. after the addition of 175 mL of 0.100M HCl.____________

Answer 1

a) Intially, there is only ammonia in the titration flask. Consider the dissociation of ammonia.

We can calculate the concentration of OH- based on the kb value and then the pOH value.

Since we know the pOH, we can calculate the PH now.

b) At the mid point half of the amount of ammonia present is neutralized. We apply this knowledge into Henderson-Hasselbach equation,

c) The volume of HCl added to reach the equivalence point can be calculated as follows.

At the equivalence point, HCl neutralizes all the ammonia present. The tiration flask contains the conjugate acid of ammonia, which is NH₄+. The acidic salt undergoes hydrolysis as below,

The hydrolysis constant for the above reaction can be defined as,

The concentration of NH₄+ can be calculated using,

We can calculate the concentration of H+ and then the pH using the equation for the hydrolysis of NH₄+ .

d) The excess concentration of HCl,

The pH of the solution,