What is the pH of the buffer after the addition of 10.0 mL of 0.10 M HCl to 72.5 mL?

Question

to 0.20 M NH3/0.17 M NH4Cl buffer solution

i am getting 9.87 but am being told it wrong what did you get

Answer 1

Determine the number of moles of HCl that you added. Calculate how many moles of NH3 and NH4+ you had in the initial volume of the buffer.

ADD the moles of HCl added to the NH4+ moles and SUBTRACT that same number from the NH3 moles.

Use Henderson-Hasselbalch equation to calculate new pH with new concentrations of acid and base.