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 CH 3NH 2 and 0.25 M CH 3NH 3Cl. Kb of CH3NH 2 = 6.8 x 10 -5. a. 8.67b. 9.75c. 7.42d. 9.81e. 6.78

FREE Expert Solution

We’re being asked to calculate the pH of a CH3NH2/CH3NH3Cl buffer solution after the addition of HCl solution. The buffer solution is made up of 0.25 M CH3NH3Cl and 0.25 M CH3NH2

CH3NH3Cl dissociates completely into: CH3NH3+ Cl

  • CH3NH3+ is a positive amine so it is the conjugate acid
  • This also means that: # of moles CH3NH3Cl = # of moles of CH3NH3+
  • The weak base is CH3NH2

Since the solution is a buffer, we know that CH3NH3+ is a weak acid and CH3NH2 is the conjugate base. HCl will react with the base CH3NH2

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

Reaction:         CH3NH2(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 CH 3NH 2 and 0.25 M CH 3NH 3Cl. Kb of CH3NH 2 = 6.8 x 10 -5

a. 8.67

b. 9.75

c. 7.42

d. 9.81

e. 6.78