Cliffs AP Chemistry, 3rd Edition

The analysis of NH 3 began by adding excess HCl to the synthesized compound according to
the following reaction:

Ni NH (^32) nCl s nH O (^3) aq! nNH (^4) aq Ni aq nH O 2 Cl aq
2
++++ 2
^ + + +-
h ]g ]]]ggg^ h ] g
The student was not told the coordination number of the compound, so “n” was used to repre-
sent the number of ammonia ligands surrounding the nickel(II) ion. After the reaction has
reached completion, additional HCl was then titrated back with a standardized NaOH solution:
HO (^32) ()+-aq+OH()aq" 2 HO()l
At the equivalence point, the point at which the moles of OH−equals the moles of H 3 O+, an in-
dicator mixture of bromcresol green and methyl red with a suitable end point, was used to de-
termine the equivalence point (pH of 5.1). The reason that the equivalence point is at 5.1 is that
the ammonium ion present as a product hydrolyzes resulting in an acidic solution:
NH 42 +]aqg++H O]laqg?NH (^33) ] g H O+]aqg
To determine the mass percent of Ni2+in the compound one takes advantage of the fact that the
[Ni(NH 3 )n]2+ion absorbs light. To determine the wavelength of light, which this ion absorbs,
the analytical wavelength, λmax, was determined. Fifty mL aliquots of standardized solutions
of [Ni(NH 3 )n]2+were made up by mixing nickel(II) sulfate hexahydrate in water with excess

NH 3 and the absorbances plotted. The reason that NiSO 4 ⋅ 6 H 2 O was used to provide the

[Ni(NH 3 )n]2+was that its molecular weight is known. Furthermore, SO 4 2–is less likely to disturb

the [Ni(NH 3 )n]2+coordination than would Cl–and the NiCl 2 ⋅6 H 2 O deliquesces. The absorbing

species, [Ni(NH 3 )n]2+, is identical in both the known and unknown solutions, therefore, the mo-
lar absorptivity, ε,would be the same. The molar ratio of Ni2+between Ni(NH 3 )nCl 2 and NiSO 4

⋅6 H 2 O is 1:1, making direct comparisons possible.

Data:

I. Synthesis

a. Mass of NiCl 2 ⋅6 H 3 O used = 7.00 g

b. Mass of (impure) synthesized Ni(NH 3 ) nCl 2 produced = 5.50 g

II. Mass Percent of NH 3 in Ni(NH 3 )nCl 2

a. molarity of NaOH solution = 0.100 M

b. molarity of HCl solution = 0.250 M

c. volume of HCl solution added = 25.0 mL

d. volume of NaOH solution required = 29.6 mL

e. mass of Ni(NH 3 ) nCl 2 used in titration = 0.130 g

Part III: AP Chemistry Laboratory Experiments