Ji Zhang
11-9-1998
per 1 Chem
Decomposition, formation, and Reaction of a Hydrate

Objectives:
	The objectives of this lab are to determine the percentage of water in a hydrate; to determine the 
formula of a hydrate from analytical data; and to observe color and energy changes which occur during the 
formation and reaction of hydrate.

Data:

Percentage of Water in a Hydrate








1
Mass of crucible, cover, & hydrated barium chloride




23.50g


2
Mass of crucible & cover




20.54g


3
Mass of hydrated barium chloride




2.96g


4
Mass of crucible, cover, and anhydrous barium chloride




23.04g


5
Mass of crucible and cover




20.54g


6
Mass of anhydrous barium chloride




2.50g


7
Mass of water in the hydrated crystals




.46g


8
Number of moles of anhydrous barium chloride in sample




.0120g


9
Number of moles of water in hydrated crystals




.0255g


10
Percentage of water in hydrate (experimental)




15.54%



 = 0.46/0.296







11
True percentage of water in hydrate (theo.)




14.75%



 = 36/(137+71+36)







12
Percent deviation of exp. value from theo. value




4.83%



 =exp. error/theo. *100% = (15.54-14.75)/15.54







13
Moles of water per mole of anhydrous BaCl2 (exp.)




2 moles



 .46g H2O * 1 mole

=
.0255 moles H2O 






------------








18g H20







2.5g BaCl2 * 1 mole

=
.0120 moles BaCl2






------------








137g+71g







.0120/.0120moles BaCl2  +  .0255/.0120moles H2O = 1 mole BaCl2 + 2 moles H20







14
Formlua of Hydrate




BaCl2 * 2H2O











Formation and Reaction of a Hydrate








1
Heat CuSO4 * 5H2O


color changes to cyan




2
Add H2O to CuSO4


color changes from cyan to darker blue





 + energy (evaporation of H2O)


color changes from blue back to cyan




3
Add NH3 to CuSO4 * 5H2O


color changes to purplish blue, precipitate forms




4
Add H2SO4 to Cu(NH3)4  2+


color changes back to green & solution is aqueous again





Observations:

	For the first part of the lab (determining the percentage of water in a hydrate), a white, crystal, and 
granular solid, (the hydrated barium chloride) was put in the crucible.  After heating, and evaporating the 
water in the hydrated barium chloride, the original white granular crystals became harder and shinier.  The 
volume of the solid also was smaller.  It seems to have been “pulled in” from the sides and contracted 
because the water evaporated.  

Conclusion questions:

1.	The purpose of heating the crucible before weighting is to dry the residual water in it and also to 
rid any tampering material in it.
2.	The crucible was cooled before weighing because a hot crucible weighs less because of the rising 
hot air currents created by its heat.
3.	In order to ensure the removal of all the water from the hydrate, the substance would have to be 
heated to a constant weight.
4.	Hydrated BaCl is white granular crystals while anhydrous BaCl is harder and shinier.
5.	The equation for the decomposition of hydrated barium chloride (experimental) is: (in moles)
	.0120BaCl * .0255H2O(s) he?at	.0120BaCl(l) + .0255H2O(g)
6.	This equation using whole number coefficients is:
	BaCl * 2H2O(s) ? BaCl(s) + 2H2O(g)
7.	Supposed the sample contained a volatile impurity .  This would make the percentage of water too 
high.
8.	There are possible sources of error:
	If the substance was underheated, then not all of the water will evaporate out of the hydrate and 
the H2O percentage in the end will be too low.
	If material was lost (possibly spattered out during the reaction), then it will seem that the hydrate 
lost more mass, therefore making the H2O percentage in the end too high.
9.	In part II, the evidences indicating a reaction between Cu+2 ions and water is that the color 
changed from cyan to blue and heat was given off.
10.(a)	The chemical system possed less energy after water was added to anhydrous copperII sulfate 
because when things stick together (synthesis), it is usually exothermic (energy given off).  Also, since the 
solution felt hotter when water was added, an exothermic reaction is indicated. 
(b)	The observed energy change can be accounted for since exothermic reactions usually occur when 
reactants stick together (synthesis).
11.	The evidence of NH3 reacting with Cu(H2O)4+2 is that the color of the solution changed from 
blue to purplish blue when the NH3 was added.
12.	The evidence that the concentration of Cu(NH3)4_2 changed when sulfuric acid was added is that 
the purplish blue solution turned into a blue solution.
13.	The equation for the reaction between hydronium ions and ammonia molecules is:

H3O 	+ 	NH3 	? 	NH4 	+ 	H2O