What is the pH of a 0.29 M solution of methylammonium
chloride, CH3NH3Cl?
What is the concentration of methylamine in the solution?
CH3NH3Cl undergoes dissociation to form CH3NH2 and H+(aq) and the balanced chemical reaction is
------------ CH3NH3Cl(aq) -------> CH3NH2 + H+(aq) + Cl-(aq); Ka = 2.4x10-11
init.conc: 0.29 M ------------------- 0 M, ------- 0 M
change: - 0.29y M, --------------- + 0.29 y M, + 0.29 y M
eqm.conc:0.29(1 - y)M, ---------- 0.29 y M, 0.29 y M
Ka = 2.4x10-11 = [CH3NH2]x[H+(aq)] / [CH3NH3Cl(aq)]
=> 2.4x10-11 = (0.29y x 0.2y) / 0.29(1 - y)
y <<1, (1 - y) is nearly equals to 1.
=> y = underroot(2.4x10-11 / 0.29) = 9.10x10-6 M
Hence [H+(aq)] = 0.29y = 0.29 x 9.10x10-6 M = 2.64x10-6 M
=> pH = - log[H+(aq)] = - log(2.64x10-6 M) = 5.58 (answer)
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