Lab #2
Name: Alina Usher
Date: September 4, 2019
Title: Determining The Molecular Formula Of Epsom Salts
Aim: In this experiment, you will be trying to find the value of x (i.e the number of moles
of water associated with one mole of MgSO4 in Epsom salts).
Materials:
1. Goggles and Lab Coat
2. 1 Boiling Tube
3. Spatula
4. Bunsen Burner Kit
5. Test Tube Holder
6. Digital balance
Chemicals:
1. Epsom Salts
Procedures:
1. A clean, dry boiling tube was weighed.
2. About 3 grams of Epsom salts were put in the boiling tube and
reweighed.
3. The weighing and those which followed were recorded in a clear table.
4. The boiling tube was then heated gently for a minute and then was
heated strongly for 5 minutes.
5. The tube was allowed to cool and was then reweighed.
6. The tube was heated again for two minutes. It was allowed to cool and
was reweighed.
7. If the last two weighing had differed by more than 0.02 g, then the
heating would be repeated and weighed again. The heating of a
substance, like Epsom salt, until its weight does not change any more
then it would be called “heating to constant weight”.
8. The procedures were repeated for trial 2, it was taken for consideration
that the Epsom salts must have been constant on both trials.
Results:
Trials Weight of Weight of Weight after Weight after
empty boiling boiling tube heating for 6 heating for an
tube (g) with Epsom minutes (gently additional 2
salt (g) and vigorously) minutes (g)
(g)
1 24.17 27.17 25.65 25.65
2 24.17 27.17 25.67 25.65
Average 24.17 27.17 25.66 25.65
WEIGHT OF BOILING TUBES WHEN EMPTY, WITHEPSOM SALT, AFTER 6
MINUTES AND AFTER ADDITIONAL 2 MINUTES RESPECTIVELY
Calculations:
Mass of Hydrated MgSO4 used: 27.17g – 24.17g = 3.00 g
Mass of water in hydrated salt (amount expelled): 27.17g – 25.65g = 1.52 g
Mass of MgSO4 in this hydrated salt: 3.00g – 1.52g = 1.48 g
Mr of water (H2O) = (1 x 2)+ 16 = 18 g/mol
Mr of Magnesium Sulphate (MgSO4) = 24 + 32 + (16 x 4) = 120 g/mol
- Mole of Magnesium Sulphate used in reaction: 1.48g / 120gmol-1 = 0.0123 mol
- Mole of Water in reaction: 1.52g / 18 gmol-1 = 0.084 mol
1 mol MgSO4 = x mol H20
0.0123 mol = 0.084 mol
Therefore .084 / 0.0123= 6.83 ; to the nearest whole number = 7 mol
1 mol MgSO4 = 7 mol H20
Percentage Error 7- 6.83 =0.17
- 0.17 / 7 * 100 = 2.4%
Discussion: The molecular formula is defined as a formula giving the number of atoms of
each of the elements present in one molecule of a specific compound. A hydrated salt is a
crystalline salt molecule that is loosely attached to a certain number of water molecules.
Salt is created when an acid's anion and a base's cation are combined to produce an acid-
base molecule. The word “anhydrous” in itself means without water. An anhydrous salt is
only the cation and anion making up the salt. There is no water of hydration.
Many salts naturally draw water from the environment and occur in nature as hydrated salt.
Water of crystallization or water of hydration are water molecules that are present inside
crystals. Water is often incorporated in the formation of crystals from aqueous solutions. In
the experiment, as the Epsom salt becomes heated, the crystal structure changed. Well-
formed crystals are usually given off but may shatter and form a powder as the water of
hydration is driven off, A chalk-looking and powdery substance was observed and it was
shown that water was given off due to water droplets being seen at the top of the boiling
tube. The Epson salt was heated to a constant weight in order to remove the water of
hydration, the loss of weight is the weight of water present in the original salt. From my
results, 1.52g of H20 combine with 1.48g of anhydrous MgSO4 in Epsom salts. There were
0.084 moles of H20 combine with 0.0123 mol of anhydrous MgSO4 in Epsom Salts. As
calculated, 7 moles of H20 combine with 1 mole of anhydrous MgSO4. We would use
MgSO4.7H20 as the theoretical formula and the precise formula is MgSO4.6.83H2O
Source of error: A possible source of error may be due to the water droplets inside the
boiling tube dropping into the anhydrous salt rather than evaporating. This would cause the
mass of the Epson salt to differ by more than it should.
Conclusion: The masses and number of moles for both magnesium sulphate and water was
successfully found. Those value were used to calculate x. The value of x, which is the
number of moles of water associated with one mole of MgSO4 was found to be 6.83 which
was off by 2.4% .
