ACIDS AND BASES Calculating the composiltion of a buffer of a given pH A chemist

Question

ACIDS AND BASES Calculating the composiltion of a buffer of a given pH A chemistry graduate student is given $00 ml. of a 1.00 M diethylamine ((C,H,), NH) solution mass of (C,H,),NH,CI should the student dissolve in the (C,H.,),NIH solution to turn t into a buffer with pH -10.9 Diethylamine is a weak base with Kk,-13-10 w You may assume that the volume of the solution doesn't change when the (C,H,) NH, CI is dissolved in it. Be sure your answer has a unt symbol, and round it to 2 significant digits Check

Explanation / Answer

The pKb of diethylamine = -Log(Kb)

= -Log(1.3*10-3)

= 2.886

Therefore, pKa of diethylamine = 14 - 2.886 = 11.114

The no. of mmol of diethylamine (ndiethylamine)= 500 mL * 1 mmol/mL = 500 mmol

According to Henderson-Hasselbulch equation

pH = pKa + Log(ndiethylamine/ndiethylammonium chloride)

i.e. 10.91 = 11.114 + Log(500/ndiethylammonium chloride)

i.e. Log(niethylammonium chloride/?500) = 11.114 - 10.91 = 0.204

i.e. ndiethylammonium chloride/?500 = 100.204 = 1.6

i.e. ndiethylammonium chloride = 500*1.6 = 800 mmol

i.e. Mass/109.5 g mol-1 = 800*10-3 mol

i.e. The required mass of diethylammonium chloride = 800*10-3 mol * 109.5 g mol-1 = 87.6 g

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