Consider the titration of 300.0 ml of 0.425 M NH₃ (Kb = 1.8 × 10⁻⁵) with 0.500 M HNO₃. After 150.0 ml of 0.500 M HNO₃ have been added, the pH of the solution is:

a) 4.74
b) 5.29
c) 8.23
d) 9.16