Composition of a Hydrate
Introduction:
Hydrates are salts or ionic compounds that have a specific amount of water as part of the
structure. The water is chemically bound to the salt in a definite small whole number ratio. The
amount of water present depends on the type of salt. The word hydrate means ” with water.”
http://en.wikipedia.org/wiki/Hydrate
The formula of a hydrate is written in a specific way to denote the number of waters that
bond with each molecule. The compound has a _____ ∙ x H2O after its name. The ∙ indicates
that the water is loosely held to the salt. The x stands for the number of water molecules bonded
to the one unit of the salt. The prefixes for the number of water molecules bonded to the salt are
listed below.
Prefixes for the number of Water molecules attached to a salt
Monohydrate
1
Hexahydrate
6
Di-
2
Hepta-
7
Tri-
3
Octa-
8
Tetra-
4
Nona-
9
Penta-
5
Deca-
10
If a compound has 2 water molecules as part of the hydrate it would be:
BaCl2∙2H2O Barium chloride dihydrate
When hydrates are heated, water is released as a vapor or gas. The remaining solid is known as
the anhydrous salt.
hydrate  anhydrous salt + water
If the mass of the hydrate is compared to the mass of the anhydrous salt after heating, the
difference in the mass is due to the loss of water.
The percent of water in a hydrate can be determined by the following formula.
% 𝑤𝑎𝑡𝑒𝑟 =
𝑤𝑎𝑡𝑒𝑟
× 100%
ℎ𝑦𝑑𝑟𝑎𝑡𝑒
In this experiment, a hydrate of copper sulfate will be studied (CuSO4∙x H2O). It will have an
unknown quantity of water. The task is to determine the exact percentage of water and formula
for the hydrate. The change of the hydrate to the anhydrous salt occurs with a color change.
Purpose:
Determine the percentage of water in a hydrate and the formula for the hydrate.
Equipment/Materials:
Porcelain evaporating dish
wire gauze
ring stand
Balance
Insulated square
Bunsen burner
Crucible tongs
Copper sulfate hydrate: CuSO4∙x H2O
iron ring
Goggles
Micro spatula
Apron
Procedure:
1. Prepare the setup as shown.
2. Heat the evaporating dish with the hottest part of the flame for 3 minutes.
3. Using crucible tongs, remove the evaporating dish from the apparatus. Place it on an
insulated pad and allow it to cool for 3 minutes.
4. Find the mass of the evaporating dish. Record the mass in the table.
5. With the evaporating dish on the balance, measure approximately 2.00 g of copper sulfate
hydrate into the dish. Record the mass in the table.
6. Place the evaporating dish and hydrate on the wire gauze. Gently heat the dish by moving
the burner back and forth around the base. Increase the heat gradually. Avoid any popping
and spattering.
7. Heat strongly for 5 minutes or until the blue color has disappeared. During heating. A micro
spatula may be used to break up any caked portions and spread the solid around. If the edges
of the solid start to turn brown, remove heat momentarily, then resume heating at a gentler
rate.
8. Cool the evaporating dish for 1 minute. Immediately find the mass of the dish and anhydrous
salt. Record in the data table.
9. Calculate % water in the hydrate. When calculations are complete, place % water value on
the board.
10. Wait 5 minutes, find the mass and anhydrous salt. Record in the data table.
11. Wait another 10 minutes, find the mass and anhydrous salt.
12. Take a small beaker (50-mL), split the anhydrate into two portions. One portion into a small
beaker, and the remaining portion stays inside the evaporation dish.
13. Use a disposable pipette; put two drops of distilled water into the evaporation dish containing
the anhydrate. Observe.
14. For the portion in the small beaker, put the beaker aside on the bench overnight or for the
weekend. Observe and measure the total mass (mass of the beaker + mass of the anhydrate).
15. Repeat step 14 one more time. Observe and measure the total mass.
Pre-lab Questions:
1. Why the empty evaporation dish has to be heated prior to the measurement of mass?
2. What are the assumptions made for this lab?
3. How to calculate the proper formula for the hydrate?
4. How to calculate % error in measurements?
5. Construct proper data tables.
Observations:
Calculations:
1.
Experimental % water: _______________
2.
Class average using the values on the board
Questions:
_______% water
1.
What is the experimental error for the % water?
2.
Why did the evaporating dish need to cool before measuring the mass?
3.
Why must the anhydrous salt be massed immediately upon a brief cooling?
4.
Briefly discuss any differences or similarities of the class average compared to your group’s
experimental % of water.
5.
If there were 7 water molecules bonded into a hydrate, what would it be called?
__________________________
6.
If there were 1 water molecule bonded into a hydrate, what would it be called?
__________________________