Question

A chemistry graduate student is given 100 mL of a 0.60 M trimethylamine ((CH), N solution. Trimethylamine is a weak base with K - 7.410 mass of (CH), NHCI should the student dissolve in the (CH), N solution to turn it into a buffer with pH - 11.067 . What You may assume that the volume of the solution doesn't change when the (CH), NHCl is dissolved in it. Be sure your answer has a unit symbol, and round it to 2 significant digits. . x 5 ?

Answer 1

Kb = 7.4*10^-4

pKb = - log (Kb)

= - log(7.4*10^-4)

= 3.131

POH = 14 - pH

= 14 - 11.06

= 2.94

use formula for buffer

pOH = pKb + log ([(CH3)3NHCl]/[(CH3)3N])

2.94 = 3.1308 + log ([(CH3)3NHCl]/[(CH3)3N])

log ([(CH3)3NHCl]/[(CH3)3N]) = -0.1908

[(CH3)3NHCl]/[(CH3)3N] = 0.6445

[(CH3)3NHCl]/0.6 = 0.6445

[(CH3)3NHCl] = 0.3867

volume , V = 1*10^2 mL

= 0.1 L

use:

number of mol,

n = Molarity * Volume

= 0.3867*0.1

= 3.867*10^-2 mol

Molar mass of (CH3)3NHCl,

MM = 3*MM(C) + 10*MM(H) + 1*MM(N) + 1*MM(Cl)

= 3*12.01 + 10*1.008 + 1*14.01 + 1*35.45

= 95.57 g/mol

use:

mass of (CH3)3NHCl,

m = number of mol * molar mass

= 3.867*10^-2 mol * 95.57 g/mol

= 3.696 g

Answer: 3.7 g