Question

1. If 0.35 mole of ammonia, 0.10 mole of ammonium nitrate, and 0.20 mole of nitric acid
are added to one liter of water, what is the final ammonia/ammonium mole ratio in
solution?

2.) 20.0ml of 1.0M NaOH is added to 200.0ml of a 0.40M NH3 and 0.50M NH4NO3
solution. By this addition, the pH should change from _____ (no NaOH) to ______(after
the addition of strong base to the buffer).

Answer 1

Q1) It is a buffer solution of NH3 and ammonium ion.and some acid added to it

NH3 + H+ --------------> NH4+

0.35 0.20 0.10 added moles

0.15 0 0.30 after reaction

Thus after reaction the ratio of NH3 to ammonium is = 0.15/0.30 = 1/2

Q2) the buffer is NH3 and NH4+ to which base is added

NH4+ + OH- --------------> NH3 + H2O

200x 0.5 0 200x0.4 - initial mmoles

- 20x1.0 - - change

80 0 100 - after reaction

The pH of buffer is calculated using Hendersen equation as

pH = 14-[pkb + log [conjugate acid]/[base]]

pKb of NH3 = 4.75

initial pH (before addition of base)

pH = 14 - [4.75 + log 100/80]

= 9.153

after adition of NaOH

pH = 14- [4.75 + log 80/100]

= 9.347

Thus the ph changes from 9.15 to 9.35 after the addition of strong base.