What is the pH?

Question

Hey i'm a little confused with this question..can someone help me out? Thanks in advance!

Consider a weak base-strong acid titration in which 25.0mL of 0.100 M ammonia is titrated with 0.100 M HCl.

a) What is the pH of the solution before the addition of HCl? (pka of ammonia = 4.75)

b) Calculate the pH after the addition of 3.00 mL of HCl

Answer 1

The first one's not quite so easy. First, convert pKa into Ka. Then calculate Kb which is 10^-14/Ka.

Now, when you dissovle NH3 is water, it reacts with water to form ammonium ions and OH- ions. But this really happens to a very slight extent. So,

Kb = [NH4+][OH-]/[NH3]
The [NH4+] and [OH-] will be equal so let them both be x. The [NH3] will be 0.100 M.

Solve the equation for x which is the [OH-]. Are you OK converting a hydroxide ion concentration into a pH?

Now, when you've added 3 mL of HCl, the concentration of HN3 will be decreased by the number of moles of HCl you added, and the concentration of ammonium ions will be equal to the number of moles of HCl you added.

For part b, you can use the Henderson-Hasselbalch equation :

pH = pKa + log([base]/[acid] where [base] is the remaining ammonia concentration, and [acid] is the ammonium ion concentration.

Hope this helps...If not, send me a message.