In: Chemistry
Consider the titration of 50.0 mL of 0.20 M NH3 (Kb=1.8×10−5) with 0.20 M HNO3. Calculate the pH after addition of 50.0 mL of the titrant.
NH3 millimoles = 50 x 0.20 = 10
millimoles of HNO3 = 50 x 0.20 = 10
NH3 + HNO3 -------------------> NH4+NO3-
10 10 0 -------------------> initial
0 0 10 -------------------> after reaction
here salt NH4+NO3 only remains in the mixture.
so pH is decided by salt hydrolysis of weak base strong acid salt
now
pKb of NH3 = 4.75
molarity of salt = millimoles / total volume
= 10 / (50+50)
= 0.1 M
C= 0.1 M
pH = 7 - 1/2 [pKb + logC]
pH = 7 - 1/2 [4.74 + log 0.1]
pH = 5.13