What is the best way to calculate pH here?

Question

When mixing 40 mL of 0.1 M NH3 and 20 mL of 0.25M HCl.

I have the answer and how to do it but the way it is done is very very vague. I know in this case the acid is in excess compared to base. But would I calculate the same problem if there was excess base? What if the amounts were equal?

Answer 1

If the acid is in excess, ignore the ammonium chloride. Just work out the surplus moles of HCl, and divide by the total volume.

If the base is in excess, then you have a classic buffer solution calculation, and you have to work out the concentration of both NH3 and (NH4)+.

If the number of moles of HCl and NH3 are equal, then your problem is once again different, and you have to work out the (acidic) pH of the salt NH4Cl.