Concentration pH Kp questions?

Hydrochloric acid is used to titrate 25.0 mL of 0.0100 M NH3. What is the pH after the addition of 40.0 mL of 0.0100 M HCl? (Kb for NH3 = 1.8 ยด 10-5)


a. 2.64
b. 3.82
c. 4.12
d. 4.94
e. 5.00

Answer:
moles NH3 = 0.0100 x 25.0 /1000 = 0.000250

moles HCl = 0.0100 x 40.0 /1000 = 0.000400

moles H+ in excess = 0.000400 - 0.000250 = 0.000150

total volume = 65 mL = 0.065 L

[H+] = 0.000150 / 0.065 = 0.00231 M

pH = -log 0.00231 = 2.64

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