Question

Calculate the pH of a solution that results from mixing 27 mL of 0.13 M methylamine (CH₃NH₂) with 27 mL of 0.11 M CH₃NH₃Cl. The K_b value for CH₃NH₂ is 3.6 x 10^-4.

Answer 1

Answer – Given, [CH₃NH₂] = 0.13 M , volume = 27 mL , [CH₃NH₃Cl] = 0.11 M

Volume = 27 mL , Kb = 3.6 x 10^-4

We need to calculate the first pKb

pKb = -log Kb

        = -log 3.6 x 10^-4

        = 3.44

We need to use the Henderson Hasselbalch equation –

pOH = pKb + log [CH₃NH₃Cl] / [CH₃NH₂]

         = 3.44 + log 0.11 M / 0.13 M

          = 3.37

We know,

pH = 14 – pOH

      = 14 – 3.37

      = 10.6