Methylamine is a weak base for which kb is 4.4*10 −4 . calculate the ph of a solution made by dissolving 0.070 mol of methylamin

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4 years ago

11

e in water and diluting to 800 ml.

1 answer:

SVEN [57.7K] · Answer 1 · 2022-01-30T11:09:16+03:00

Answer is: pH of methylamine is 11.78.

Chemical reaction: CH₃NH₂(aq)+ H₂O(l) ⇌ CH₃NH₃⁺(aq) + OH⁻(aq).
Kb(CH₃NH₂) = 4,4·10⁻⁴.
c₀(CH₃NH₂) = n(CH₃NH₂) ÷ V(CH₃NH₂).

c₀(CH₃NH₂) = 0.070 mol ÷ 0.8 L = 0.0875 M.

c(CH₃NH₃⁺) = c(OH⁻) = x.

c(NH₂OH) = 0.0875 M - x; equilibrium concentration of methylamine.

Kb = c(CH₃NH₃⁺) · c(OH⁻) / c(CH₃NH₂).

0.00044 = x² / (0.0875 M - x).

Solve quadratic equation: x = c(OH⁻) = 0.00598 M.

pOH = -log(0.00598 M) = 2.22.

pH = 14 - 2.22 = 11.78.