Cliffs AP Chemistry, 3rd Edition

She allowed it to cool in a desiccator (see Figure 2),

Figure 2

and then reweighed it and discovered it weighed 1.006 grams. The anhydride produced was
yellow.

Analysis:

1. Describe the processes occurring with the cobalt chloride.

Three color changes were observed: blue, violet and red. From the information provided,

the blue form appeared to be the anhydride, CoCl 2. When exposed briefly to air containing

water vapor, the anhydrous cobalt chloride turned violet, CoCl (^2) ⋅2H 2 O. Upon further
standing in air, the dihydrate was converted to the fully hydrated form, CoCl (^2) ⋅6 H 2 O. The
color of the anhydride-hydrate depended upon the relative humidity of the air to which it
was exposed.

2. Describe the processes occurring with the sodium sulfate decahydrate.

It appeared that the large, well-defined crystals of sodium sulfate decahydrate, Na 2 SO (^4) ⋅
10 H 2 O, lost water when exposed to air and converted to the anhydrous form, Na 2 SO 4.
This process is known as efflorescence. It would appear that the relative humidity of the
air was rather low for this to occur. When rehydrated and gently dried, the hydrate reap-
peared indicating a reversible reaction:
Na SO 24 : 10 H O heat 2 ++)Na SO 2410 H O 2

3. What was the formula of the green nickel sulfate hydrate?

.

.

gH O

g NiSO

0 694

1 006

42

2

• 4

1 700.?g NiSO : H O

.

..

g NiSO

g NiSO

mol NiSO mol NiSO

1

1 006

154 76

(^41) 0 006500
4
#^4 = 4
To ensure a good
seal, lubricate the
lip of the desiccator
with stopcock grease.
Desiccant such as
anhydrous calcium
chloride. A desiccator provides a low humidity
environment. Never place hot objects
inside a desiccator since they may
create a partial vacuum resulting
in spilling the sample when the
desiccator is opened.
Laboratory Experiments