Question

5.00 mL of 0.250 M ammonia (NH3) is titrated with 0.100 M hydrochloric acid (HCl). The...

  1. 5.00 mL of 0.250 M ammonia (NH3) is titrated with 0.100 M hydrochloric acid (HCl). The Kb for ammonia is 1.75 x 10-5
    What is the pH of the ammonia solution prior to the addition of any HCl?
    What is the pH of the solution after 5.00 mL of HCl has been added?
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Answer #1

initially only NH3 present.

for weak acid

pOH = 1/2 [pKb - log C]

pKb = - log Kb = - log [1.75 x 10-5] = 4.76

pOH = 1/2 [4.76 - log 0.250]

pOH = 2.68

pH = 14 - 2.68

pH = 11.32

2) initially

millimoles of NH3 = 5.00 x 0.250 = 1.250

millimoles of HCl added = 5.00 x 0.100 = 0.50

after HCl added

millimoles of NH3 left = 1.250 - 0.50 =0.750

millimoles of NH4+ = 0.50

total volume = 5.00 + 5.00 = 10.00 mL

[NH3] = 0.750 / 10.00 = 0.075 M

[NH4+] = 0.50 / 10.00 = 0.05 M

pOH = pKb + log [NH4+] / [NH3]

pOH = 4.76 + log [0.05] / [0.075]

pOH = 4.58

pH = 14 - 4.58

pH = 9.42

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