CHM 100 LAB 7 HYDRATES AND THE MOLE
INTRODUCTION
This laboratory exercise investigates hydrated compounds. The mass and mole relationships
between the anhydrous and water will be investigated. Also the measured and calculated
relationships between gram, mole, atoms, molecules or particles of different materials will be
investigated.
INVESTIGATING A HYDRATED COMPOUND
When an ionic solid is crystallized from water solution, the crystal which forms often contains
chemically-bound water molecules. The number of moles of water per mole of ionic substance is
usually an integer. Compounds of this sort are called hydrates. Among the commonly encountered
hydrates are the following:
CuSO4 . 5H2O
MgSO4 . 7H2O
BaCl2 . 2H2O
Formulas for hydrates are written using a dot convention: a dot is used to separate the formula of the
salt from the formula of the water of hydration. A numerical coefficient gives the molar amount of
water included in the hydrate. Hydrates are named using prefixes for the word hydrate (at right). For
example, CuCl2·2H2O is copper (II) chloride dihydrate and CuSO4·5H2O is copper (II) sulfate
pentahydrate. One key point: the dot is not a multiplication sign. When calculating the molar mass
you add the molar mass of water (multiplied by the coefficient).
An everyday example of hydration is concrete. Concrete is made by mixing Portland cement with
water and aggregate materials. The aggregate materials are the gravel and sand that add strength to
the final concrete. The Portland cement is a mixture of calcium silicates, calcium aluminate, calcium
aluminoferrite and gypsum. All of these chemicals absorb water by hydration. This means that
concrete does not ‘dry’ in a conventional sense. Instead the water mixed with the concrete combines
chemically with the materials in the cement and the resulting hydrates form a strong matrix that holds
the concrete together and makes it strong.
Another interesting example of the value of hydration is the incorporation of hydrated building
materials (such as concrete, gypsum wall board and plaster). The building materials will not rise
above the 100°C boiling point of water until all of the water of hydration has been driven off. This can
help keep damage to a minimum until the fire can be put out. In the construction business this is
known as passive fire protection.
The water in a hydrate is bound loosely, and so is relatively easily removed by heating. Most
hydrates lose their water of hydration at temperatures slightly above 100oC. Sometimes the water is
liberated in stages, with one or more lower hydrates being observed during the heating
process. Thus, CuSO4 may also be prepared with 3 moles of H2O or 1 mole of H2O per mole of ionic
solid. If all the hydrated is removed, as it will be if the solid is heated sufficiently, the ionic solid is said
to be anhydrous (without water).
Given the mass of a sample of the hydrate (with water bound in the crystal) and the mass
of anhydrous salt (without water bound in the crystal) of known formula obtained on heating, it is
easy to find the formula of the hydrate. One simply needs to determine the number of moles of water
per mole of anhydrous compound in the hydrate.
1 SAMPLE DATA
For the investigation of a hydrated compound: MgSO4 • 7H2O
Mass of empty crucible
2.3455 g
Mass of crucible and MgSO4 • 7H2O
5.2489 g
Mass of MgSO4 • 7H2O
5.2489 g - 2.3455 g = 2.9034 g
Molar mass of
1 Mg = 24.305
MgSO4 • 7H2O
1 S = 32.065
4 O = 15.9994 x 4
14 H = 1.0079 x 14
7 O = 15.9994 x 7
+
246.474 g/mol
Calculate the moles of
MgSO4 • 7H2O
Calculate the particles of
MgSO4 • 7H2O
Mass of empty crucible
Mass of crucible and MgSO4
Mass of MgSO4
Calculate the molar mass
MgSO4
2.3455 g
3.7634 g
3.7634 g -2.3455 g = 1.4179 g
1 Mg = 24.305
1 S = 32.065
4 O = 15.9994 x 4
120.3676 g/mol
+
Calculate the moles of
MgSO4
Calculate the particles of
MgSO4
Mass of MgSO4 • 7H2O
Mass of MgSO4
Calculate mass of H2O
Calculate the molar mass
H 2O
2.9034 g
1.4179 g
(Mass of MgSO4 • 7H2O- Mass of MgSO4)
2.9034 g - 1.4179 g = 1.4855 g
2 H = 1.0079 x 2
O = 15.9994
+
18.0153 g/mol
Calculate the moles of H2O
Calculate the molecules of
H 2O
Moles of MgSO4
0.011780 mol
Ratio: Ratio:
0.011780 mol/0.011780 = 1 mol MgSO4
0.082528 mol/0.011780 = 7 mol H2O
Therefore the formula matches the data: MgSO4 • 7H2OsxzaZ
Moles of H2O
0.082528 mol
For the investigation of mole quantity of different elements and compounds
Molar mass of
(58.6934) + (35.453 x 2) =129.5994 g/mol
nickel (II) chloride: NiCl2
1
Calculate the mole quantity of 5.22 x 1022 particles of
0.0867
5.22 10
NiCl2
6.02 10 129.5994
Calculate the mass quantity of 5.22 x 1022 particles of
11.2
0.0867
NiCl2
1
2 OBJECTIVES
The goals for this experiment are:
1. To investigate a hydrated compound and determine mole ratio of anhydrate to water.
2. Measure masses of sample material determined by the calculation of moles and mass.
ORIENTATION
This experiment will require the use of the following glassware and hardware:
1.
2.
3.
4.
5.
6.
7.
8.
ring stand
ring
crucible tongs
crucible
clay triangle
distilled water bottle
Bunsen burner
igniter
This experiment will use the following chemicals:
1.
2.
3.
4.
CuSO4·5H2O
zinc
aluminum oxide
dihydrogen monoxide
3