Question

Titration of Ammonia with Hydrochloric Acid

a)Find the pH of 35.00mL of 0.100M ammonia

b) Find the pH after 15.00mL of 0.100M HCL has been added to 35.00mL of 0.100M NH₃

Answer 1

a) initially only NH3 present it is weak base

for weak bases

pOH = 1/2 [pKb - log C]

pKb = 4.75

pOH = 1/2 [4.75 - log 0.1]

pOH = 2.87

pH = 14 - 2.87

pH = 11.13

b) millimoles of NH3 = 35 x 0.1 = 3.5

millimoles of HCl added = 15 x 0.1 = 1.5

NH3 + HCl -----------> NH4Cl

3.5 - 1.5 = 2.0 millimoles NH3 left

1.5 millimoles NH4Cl formed.

total volume = 15 + 35 = 50

[NH4Cl] = 1.5 / 50 = 0.03 M

[NH3] = 2.5 / 50 = 0.05 M

pOH = pKa + log [NH4Cl] / [NH3]

pOH = 4.75 + log [0.03] / [0.05]

pOH = 4.53

pH = 14 - 4.53

pH = 9.47