Question

Consider a solution of 0.36 M C₂H₅NH₂ (K_b = 5.6×10^-4). Decide if each of the following is a major or minor species in the solution, and calculate the pH.

OH⁻

H₂O

C₂H₅NH₃⁺

C₂H₅NH₂

H⁺

^{AND pH?}

Answer 1

The protonation is as follows,

C2H5NH2 + H2O ----------> C2H5NH3+ + OH-

Initially,

0.36 ......................................0..................0

Finally,

0.36 - X.................................X.................X

We know that,

Kb = [OH-] [C2H5NH3+] / [C2H5NH2]

=> Kb = 5.6×10 ^-4 = X ² / 1 - X

=> X ² + 5.6×10 ^-4 X - 5.6×10 ^-4 = 0

Solving the above quadratic for X, we get

X = 2.37 x 10 ^-2 M

=> [OH-] = X = 2.37 x 10 ^-2 M (MINOR)

H2O (MAJOR)

[C2H5NH3+] = X = 2.37 x 10 ^-2 M (MINOR)

[C2H5NH2] = 0.36 - X = 0.336 M (MAJOR)

[H+] = 10 ^-14 / [OH-] = 10 ^-14 / 2.37 x 10 ^-2 = 4.22 x 10 ^-13 M (MINOR)

pH = - log [H+] = - log (4.22 x 10 ^-13) = 12.37