CHE 141
Experiment 5
The Formula of a Hydrate
Purpose:
To determine the formula of a hydrated compound by heating
Background Information:
Hydrates are crystalline solids that contain a fixed number of water molecules as an
integral part of their crystalline structure. A common hydrates is copper(II) sulfate
pentahydrate, which contains 5 moles of water per 1 mol of copper(II) sulfate, written as
CuSO4·5H2O. It is used as a catalytic precursor, fungicide, and as a source of copper in
chemical manufacturing processes.
Many hydrates can be transformed to the anhydrous compound when heated
strongly. For example, copper sulfate pentahydrate can be converted into anhydrous
copper sulfate. This change can be followed visually. The blue crystalline copper sulfate
pentahydrate is converted when heated to a white, powdery, anhydrous salt, according
to:
CuSO4·5 H2O → CuSO4 + 5 H2O
blue white
Or generally
heat
hydrated salt anhydrous salt + water vapor
It is also possible to reverse the above process, as shown in the equation below:
CuSO4 + 5 H2O → CuSO4·5 H2O
white blue
If water is added to the white anhydrous copper sulfate, a blue color is obtained indicating
that the blue pentahydrate is regenerated. The property of reversibility can be used to
distinguish true hydrates from other compounds that produce water when heated.
Since many hydrates contain water in a stoichiometric quantity, it is possible to determine
the molar ratio of water to salt. A sample an accurately weighed sample of the hydrate
will be heating to drive out the water. The compound formed is now anhydrous. By
determining the mass of the anhydrous sample and subtracting this mass from that of the
hydrate, we can determine the amount of water in the original substance.
, Sample Calculation
A 3.90 g sample of CuSO4 hydrate was heated and 2.50 g of the anhydrous salt
remained. The following data was obtained.
Data Collected
Mass of clean crucible and lid 22.36 g
Mass of crucible, lid and hydrate 26.26 g
Mass of hydrate 3.90 g
Mass of crucible, lid and anhydrous salt 24.86 g
(after heating)
Mass of anhydrous salt 2.50 g
Mass of water evaporated 1.40 g
Calculations to Moles
2.50 g CuSO4 1 mol CuSO4 = 0.0157 mole CuSO4
159.0 g CuSO4
1.40 g H2O 1 mol H2O__ = 0.0777 mole H2O
18.02 g H2O
Stoichiometric Ratio
Moles of Water_______ = 0.0777 mol H2O = 4.95
Moles of Anhydrous Salt 0.0157 mol CuSO4
Formula of the Hydrate
CuSO4 . 5 H2O
Experiment 5
The Formula of a Hydrate
Purpose:
To determine the formula of a hydrated compound by heating
Background Information:
Hydrates are crystalline solids that contain a fixed number of water molecules as an
integral part of their crystalline structure. A common hydrates is copper(II) sulfate
pentahydrate, which contains 5 moles of water per 1 mol of copper(II) sulfate, written as
CuSO4·5H2O. It is used as a catalytic precursor, fungicide, and as a source of copper in
chemical manufacturing processes.
Many hydrates can be transformed to the anhydrous compound when heated
strongly. For example, copper sulfate pentahydrate can be converted into anhydrous
copper sulfate. This change can be followed visually. The blue crystalline copper sulfate
pentahydrate is converted when heated to a white, powdery, anhydrous salt, according
to:
CuSO4·5 H2O → CuSO4 + 5 H2O
blue white
Or generally
heat
hydrated salt anhydrous salt + water vapor
It is also possible to reverse the above process, as shown in the equation below:
CuSO4 + 5 H2O → CuSO4·5 H2O
white blue
If water is added to the white anhydrous copper sulfate, a blue color is obtained indicating
that the blue pentahydrate is regenerated. The property of reversibility can be used to
distinguish true hydrates from other compounds that produce water when heated.
Since many hydrates contain water in a stoichiometric quantity, it is possible to determine
the molar ratio of water to salt. A sample an accurately weighed sample of the hydrate
will be heating to drive out the water. The compound formed is now anhydrous. By
determining the mass of the anhydrous sample and subtracting this mass from that of the
hydrate, we can determine the amount of water in the original substance.
, Sample Calculation
A 3.90 g sample of CuSO4 hydrate was heated and 2.50 g of the anhydrous salt
remained. The following data was obtained.
Data Collected
Mass of clean crucible and lid 22.36 g
Mass of crucible, lid and hydrate 26.26 g
Mass of hydrate 3.90 g
Mass of crucible, lid and anhydrous salt 24.86 g
(after heating)
Mass of anhydrous salt 2.50 g
Mass of water evaporated 1.40 g
Calculations to Moles
2.50 g CuSO4 1 mol CuSO4 = 0.0157 mole CuSO4
159.0 g CuSO4
1.40 g H2O 1 mol H2O__ = 0.0777 mole H2O
18.02 g H2O
Stoichiometric Ratio
Moles of Water_______ = 0.0777 mol H2O = 4.95
Moles of Anhydrous Salt 0.0157 mol CuSO4
Formula of the Hydrate
CuSO4 . 5 H2O
