Question

For the reaction: CH3NH2(aq) + H2O(aq) ⇌ CH3NH3 +(aq) + OH- Determine the change in the pH (ΔpH) for the addition of 6.7 M CH3NH3Cl salt to a 4.7 M solution of CH3NH2, Kb=4.38 x 10-4 . View Available Hint(s) For the reaction: CH3NH2(aq) + H2O(aq) ⇌ CH3NH3 +(aq) + OH- Determine the change in the pH (ΔpH) for the addition of 6.7 M CH3NH3Cl salt to a 4.7 M solution of CH3NH2, Kb=4.38 x 10-4 . ΔpH=12.66 ΔpH=1.86 ΔpH=10.49 ΔpH=2.17

Answer 1

The change in the pH (ΔpH) is 2,17

Step-by-step explanation:

The reaction:

CH₃NH₂(aq) + H₂O(aq) ⇌ CH₃NH₃⁺(aq) + OH⁻

`kb = ([OH^(-)][CH_(3)NH_(3)^+])/([CH_(3)NH_(2)])` (1)

In equilibrium, a solution of CH₃NH₂ 4,7M produces:

[CH₃NH₂] = 4,7 - x

[CH₃NH₃⁺] = x

[OH⁻] = x

Replacing in (1):

`4,38x10^(-4) = (x^2)/(4,7-x)`

x² + 4,38x10⁻⁴x - 2,0586x10⁻³ = 0

The solutions are:

x = -0,0456 No physical sense. There are not negative concentrations.

x = 0,04515 Real answer.

The concentration of [OH⁻] is 0,04515 M.

As pOH = -log [OH⁻] And pH+pOH = 14. The pH of this solution is:

pH = 12,65

The addition of 6,7M produce this changes in concentrations:

[CH₃NH₂] = 4,656 + x

[CH₃NH₃⁺] = 6,74515 - x

[OH⁻] = 0,04515 - x

Replacing in (1) you will obtain:

x² - 6,7907x + 0,3025 = 0

Solving for x:

x = 6,74586 No physical sense

x = 0,04484 Real answer.

Thus, [OH⁻] = 0,04515 - 0,044842 = 3,08x10⁻⁴M

pOH = 3,51.

pH = 10,49

Thus ΔpH is 12,65 - 10,49 = 2,16 ≈ 2,17

I hope it helps!