[bookworm]

a) Explain why a mixture of NH3 and NH4Cl can resist changes in pH even if a small amount of HCl is added.

b) What is the minimum volume of 0.1M HCl that would cause a drastic change to the pH of a 100mL solution containing a mixture of 0.15M NH3 and 0.15M NH4Cl? Explain your answer

[Astrokel]

Hey bookworm, you have to show some attempts so we could help you out.

[bookworm]

a)
NH3 + HCl -> NH4Cl
NH4Cl + H2O -> NH4+ + Cl- (no salt hydrolysis) + H2O
NH4+ + H2O <-> NH3 + H3O+   (acidic solution)

[Astrokel]

NH3 + HCl -> NH4Cl

This is good enough. But you do need to explain how NH₃/NH₄Cl can act as buffer solution.

[bookworm]

but how do i explain the mixture of NH3 and NH4Cl that resist changes in pH by using NH3 + HCl -> NH4Cl?

[Borek]

What is pH of your solution?

How does it change when you add some small amount of strong acid?

[bookworm]

pH is not given.
By adding a little of acid, it makes the solution more acidic?

[Borek]

I am not asking about the exact pH, but about the way of calculating it. Once you know how to calculate it, you should see why the pH change is small.

http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch

It can be also explained without the Henderson-Hasselbalch equation, just using the reaction equation, although the explanation is not quantitative then.