Problem: Calculate the pH of a solution prepared by adding 20.0 mL of 0.100 M HCl to 80.0 mL of a buffer that is comprised of 0.25 M NH3 and 0.25 M NH4Cl. Kb of NH3 = 1.8 x 10-5.a. 9.17b. 9.44c. 10.82d. 7.67e. 7.91

FREE Expert Solution

We’re being asked to calculate the pH of an NH3/NH4Cl buffer solution after the addition of HCl solution. The buffer solution is made up of 0.25 M NH4Cl and 0.25 M NH3.

NH4Cl dissociates completely into: NH4+ Cl

  • NH4+ is a positive amine so it is the conjugate acid
  • This also means that: # of moles NH4Cl = # of moles of NH4+
  • The weak base is NH3

Since the solution is a buffer, we know that NH4+ is a weak acid and NH3 is the conjugate base. HCl will react with the base NH3.

HCl will release hydronium ions (H3O+) ins solution.

Reaction:         NH3(aq) + HCl(aq) → NH4+(aq) + Cl-(aq) + H2O(l)

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Problem Details

Calculate the pH of a solution prepared by adding 20.0 mL of 0.100 M HCl to 80.0 mL of a buffer that is comprised of 0.25 M NH3 and 0.25 M NH4Cl. Kb of NH3 = 1.8 x 10-5.

a. 9.17

b. 9.44

c. 10.82

d. 7.67

e. 7.91

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