How to calculate the pH of the buffer system 0.15M NH3 / 0.35 M NH4C ?

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Answer 1 · 2017-10-12T15:55:56

A buffer is an acid/base and its conjugate acid/base. In this question we're worried about ammonia and the ammonium ion of methylamide.

It's silly to not give you the K_b value for ammonia. I looked this up in an acid equilibria table: #K_b = 1.8*10^-5#

Moreover, Henderson-Hasselbach will make this a breeze (not a person, but an equation).

#pH = pK_a + log(([base])/([acid]))#

In this case,

#K_a = (10^-14)/(1.8*10^-5) approx 5.6*10^-10#

Thus,

#pH = 9.26+log(0.15/0.35)#
#therefore pH approx 8.89#