Answer is: pH of methylamine is 12,2.
Chemical reaction: CH₃NH₂(aq)+ H₂O(l) ⇌ CH₃NH₃⁺(aq) + OH⁻<span>(aq).
Kb(</span>CH₃NH₂) = 4,4·10⁻⁴.
c₀(CH₃NH₂) =
0,57 M.
c(CH₃NH₃⁺) = c(OH⁻) =
x.
c(NH₂OH) = 0,57 M - x.
Kb
= c(CH₃NH₃⁺) · c(OH⁻) / c(CH₃NH₂).
0,00044 = x² / (0,57 M - x).
Solve
quadratic equation: x = c(OH⁻) =
0,0156 mol/L.
pOH
= -log(0,0156 mol/L.) = 1,80.
pH
= 14 - 1,80 = 12,2.
M(H₂O) = 97,2 g.
n(H₂O) = m(H₂O) ÷ M(H₂O).
n(H₂O) = 97,2 g ÷ 18 g/mol.
n(H₂O) = 5,4 mol.
N(H₂O) = n(H₂O) · Na.
N(H₂O) = 5,4 mol · 6,023·10²³ 1/mol.
N(H₂O) = 3,25·10²⁴ molecules of water.
n - amount of substance.
Na - Avogadro number.
