WLHS / Chem / Monson
Name
Date
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ACTIVITY: Percent Composition of Hydrated Crystals
Crystalline compounds that retain water during evaporation are referred to as being hydrated
or are said to contain water of hydration. The ratio of moles of water to moles of compound is
a small whole number. The formula for the hydrated compound barium chloride is:
BaCl2 ● 2H2O
The dot shows that for every one molecule of BaCl2 there are two molecules of water. The amount of
water in a hydrate can be determined experimentally by heating the compound and driving the water off.
The compound with the water removed is known as anhydrous.
In this activity, you will heat a given amount of a hydrate to determine the percentage of water in the
compound.
Procedure
1) Clean a crucible. Dry the crucible by placing the crucible and cover on a clay triangle over a Bunsen burner and
heating for one minute.
2) Carefully remove the crucible using tongs, and let it cool for one minute. Mass the empty crucible and lid.
3) Obtain from your instructor a sample of hydrated crystal. Mass the crucible, lid and crystals.
4) Place the crucible on the clay triangle and set the lid slightly ajar on the top of the crucible. Heat the crucible with
hydrated crystals for 8 minutes. Using tongs, take crucible off the triangle support and let it cool for two minutes.
Mass the crucible, lid and contents.
5) Repeat step 4. If the two heat readings are within ± 0.02 g then go on to the calculations. If not, heat for a third
time for five minutes.
6) Dispose of the solid as instructed by your teacher. Leave your lab station and all equipment clean.
Data Table and Calculations: **show work for calculations!! (look for the asterisks) **
Your Hydrated Crystal formula: _____________________
1) What is the total molar mass of your formula (remember to use the number in front of the H2O)?
2) What is the molar mass of the water (USE the multiple in front of the H2O)?
3) What should be the % composition of water in the Hydrated Crystal?
Molar mass of water x 100%
Total molar mass
= % composition water
DATA: (experimental results)
Mass of hydrate crystal, crucible, and lid
____________
Mass of empty crucible and lid
____________
**Mass of HYDRATED crystal**
(show calculation)
____________
Mass of ANHYDROUS crystals and crucible / lid:
1st heating
____________
2nd heating
____________
3rd heating
(if needed)
____________
**Mass of anhydrous salt**
(show calculation)
____________
**Mass of water lost from hydrate**
(show calculation)
____________
**% water in hydrate (experimental)**
(show calculation)
____________
% water in hydrate (theoretical (“actual”)…see calculations
you did on front page) - (show calculation)
**% Error**
(show calculation)
____________