Name: ___________________
Honors Chemistry
Hydrate Lab
Lab Partner: ______________
Period: ___ Date: _________
Purpose:
In this experiment, you will study some of the properties and characteristics
of several ionic hydrates and learn their naming and formula writing systems.
Background:
Chemical compounds that contain a discrete amount of water molecules as
part of their crystalline structure are called hydrates. Hydrates occur quite
commonly among chemical substances, especially among ionic substances. Such
compounds are often prepared in, or are re-crystallized from, aqueous solutions.
Hydrates are most commonly encountered in the study of metal salts,
especially salts of the transition metals. Water is bound in most hydrates in definite
stoichiometric proportions, and the number of water molecules bound per metal ion
is often characteristic of a particular metal ion.
A very common hydrate often encountered in the lab is copper(II) sulfate
pentahydrate, CuSO4.5H2O. The word “pentahydrate” in the name of this
substance indicates that five water molecules are bound in this substance per copper
sulfate formula unit. Hydrated water molecules are generally indicated in formulas
by using a dot to separate the water molecules from the formula of the salt itself.
The naming of ionic hydrates includes the ionic portion being named based on the
rules for naming ionic compounds, the number of water molecules is indicated with
a molecular prefix, and followed with the word hydrate.
Many hydrated salts can be transformed to the anhydrous (without water)
salt with heat. For example, when a sample of copper (II) sulfate pentahydrate is
heated, the bright blue crystals of the hydrate are converted to the white, powdery,
anhydrous salt.
CuSO4.5H2O (s) CuSO4(s) + 5H2O(g)
During the heating of copper (II) sulfate pentahydrate, the water of crystallization is
clearly seen escaping as steam from the crystals.
It is also possible to reconstitute the hydrate by adding water to the
anhydrous salt. The solid will reassume the color of the hydrated salt. Anhydrous
salts are sometimes used as chemical drying agents, or desiccants, because of their
ability to combine with water from their surroundings. For example, most
electronic equipment, shoes, etc. come packed with small envelopes of desiccant to
protect them from moisture. Substances that absorb moisture and are able to be
used as desiccants are said to be hygroscopic.
Lastly, sometimes hydrates decompose when heated and form a metal oxide
rather than simply releasing their water. This is important because you should not
assume that heating will always result in the release of water and a residue of ionic
solid.
Safety:
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goggles and aprons
copper, cobalt, nickel, chromium, and barium compounds are highly
toxic. Wash hands after use and always use spatulas to transfer
chemicals.
When heating the samples they may spatter out of the crucible. To
avoid spattering heat the sample slowly with a low flame and more
vigorously after the color change has occurred to assure completion.
Procedure:
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Mass a clean dry crucible.
Add a spatula tip full (about the size of a pea) of one of the hydrates to
the crucible and mass.
Record observations of the crystals before heating.
Place the crucible in the clay triangle and begin heating with a low
flame.
IF YOU SEE YOUR SAMPLE GETTING TOO HOT
(SPATTERING OR ABOUT TO) SLIDE THE BURNER OUT
FROM UNDERNEATH AND THEN LOWER THE FLAME AND
CONTINUE
Record observations once you see the apparent color change.
When the sample is nearly dehydrated heat another minute to assure
the dehydration goes to completion.
Turn off the burner and allow the crucible to cool for 5 minutes.
Mass the cooled crucible.
Add one or two drops of water from the de-ionized water bottle to the
crucible and record observations.
Clean out the crucible by running it under water to dissolve the
hydrate and flush it down the sink with plenty of water.
Dry the crucible with paper towels.
Heat the crucible, empty, for 1 minute to dryness.
Cool the crucible.
Obtain a different crucible and repeat the procedure for the next
sample.
After washing the second crucible use the first crucible, which should
now be cool to analyze your 3rd sample.
The crucibles should be washed and paper towel dried after samples 2
and 3.
Pre Lab Questions
Name ______________________________
1) What is the objective of this lab? ________________________________________
_______________________________________________________________________
_______________________________________________________________________
2) Define the following:
Hydrate:
Anhydrous:
Hygroscopic:
3) How is the water removed from the hydrated crystal? ______________________
________________________________________________________________________
4) Do hydrates always lose water when heated? Explain!
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
5) Name or write formulas for the following hydrates. Remember, the prefixes for
the number of hydrated crystals are the same as those used for naming molecular
compounds.
Pb(NO3)2 . 4H2O
Zinc chromate trihydrate
.
Cr3(PO4)2 7H2O
Iron (III) sulfide decahydrate
NiCl2.6H2O
Cobalt (I) dichromate dihydrate
Data and Observations:
Hydrated salt
Mass of empty
crucible (g)
Mass of crucible
and sample (g)
Observations
before heating
Copper(II) sulfate
Nickel(II) sulfate
Cobalt(II) chloride
Mass of crucible
and sample after
heating (g)
Observations after
heating
Observations after
addition of water
Theoretical
chemical formula
for each hydrate
Lab Report:
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prelab questions and data table page, completed
procedure – include a heading for this section and a single sentence referring
to the procedure in the lab handout
calculations – neatly write or type this section including a heading for the
section, a heading for each calculation, the words describing the calculation,
the math for the calculation, and the final labeled answer.
i. Mass of hydrate
ii. Mass of anhydrous residue
iii. Mass of water removed
iv. Experimental % of water in the hydrate
v. Correct (theoretical) % of water in the hydrate (based on the
correct chemical formulas)
vi. Percent error based on iv and v above
conclusion – a paragraph that includes the following: a response to your
purpose, a statement of your final result, human errors, and errors beyond
your control. (there are no additional questions with the lab)