The molecule methylamine (MW 31 I)a) is highly soluble in water (1 kg/liter at 2
ID: 480393 • Letter: T
Question
The molecule methylamine (MW 31 I)a) is highly soluble in water (1 kg/liter at 25 C), while ethane (MW 30) is practically insoluble. Explain the difference. What is the pKb for methylamine? What is the pKa for methylamine? What is the structure and charge of methylamine at pH's below the pKa? This is the acid form. What is the structure and charge of methylamine at pH's above the pKa? This is the conjugate base. What is the structure and charge of methylamine at pH exactly equal to the pKa? What is the pH of a solution prepared to be 0.6 M in methylammonium chloride and 0.3 M in methylamine? Is this considered to be within the buffer range for this molecule? (h) Same question for a solution that is 0.01 M in methylammonium chloride and 0.3 M in methylamine.Explanation / Answer
Mdthylamine solubility against ethane
(a) Methylamine is highly soluble as it forms strong hydrogen bond with water, which is absent in ethane.
(b) pKb for methylamine = 3.36
(c) pKa for methylamine = 14 - 3.36 = 10.64
(d) at pH below pKa, methylamine structure is, CH3NH3+, charge +1
(e) Above this pKa, methylamine is in neutral basic form, CH3NH2, charge zero
(f) At pH equal to pKa the structure of methylamine is 1 :1 combination of CH3NH3+ and CH3NH2 with total charge of zero
(g) 0.6 M methylammonium chloride
Ka = 2.3 x 10^-11 = x^2/0.6
x = [H+] = 3.71 x 10^-6 M
pH = -log[H+] = 5.43
(h) 0.01 M methylammonium chloride
Ka = 2.3 x 10^-11 = x^2/0.01
x = [H+] = 4.8 x 10^-7 M
pH = -log[H+] = 6.32
0.3 M methylamine
Kb = 4.36 x 10^-4 = x^2/0.3
x = [OH-] = 0.0114 M
pOH = -log[OH-] = 1.94
pH = 14 - pOH = 12.06
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