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 methylamine in water and diluting to 800 ml.

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.

substituting a hydrogen atom with a halogen in a hydrocarbon:

- the single bond remains single. a is wrong.

- the bond with hydrogen is as strong as with a halogen. c is wrong

- a hydrocarbon is saturated if there is no double bond. we do not know that so d is wrong.

- carbon will always have four valence electrons. e is wrong.

by elimination, the correct answer is b. the boiling point of the new compound increases.