Determining the Formula of Hydrated Salt

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  • Topic: Magnesium sulfate, Water, Hydrate
  • Pages : 5 (1010 words )
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  • Published : November 11, 2009
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INQUIRY- What is the percentage by mass of water in Epson salt?

When a crystal forms from water solutions, it takes a certain amount of water as part of the crystalline structures. The water is not covalently bonded to the host molecule or ion. Water is taken up in definite proportion in relation to the mass of the crystalline structure. This water that is trapped inside the hydrated solid structure is called water of crystallization or water of hydration. A salt with associated water of crystallization is known as a hydrate. The formula of a hydrated salt is written as the number of moles of water present in one mole of crystalline structure.

Deliquescent materials are substances that absorb water from the atmosphere and eventually become hydrated. These materials, usually salts, tend to have a high affinity for water. During this process, the actual structure of the crystalline changes to incorporate the water molecules. This causes the crystal to change in appearance.

In order to calculate the amount of water in the crystal, the weight by difference method was used. The amount of water was calculated by measuring the differences in the mass of the hydrous and anhydrous salt. By using this mass, the percentage of water, the number of moles and the complete formula of the salt were derived.

HYPOTHESIS- If the water is driven off of hydrated magnesium sulfate, then it would be colorless because it is made of Group 2 metals, which do not form colors.

a)To use the method of “weight by difference” to determine mass quantities. b)To determine the mass of an anhydrous salt by heating the sample to a constant mass. c)To convert grams to moles.
d)To determine the percent of water in a hydrate sample.
e)To determine the formula of a hydrated compound.

2)Clay triangle
4)Crucible tongs
6)Iron ring
7)Ring stand
9)Wire gauze
10)Hydrated magnesium sulfate

The crucible was cleaned with a tissue paper in order to wipe off particles in inside it. Then the clay triangle was placed on the iron ring. The Bunsen burner was fired up and a flame was created. The crucible was place on the clay triangle and was allowed to heat gently for two minutes. The crucible was removed with a crucible tong, allowed to cool and its mass was then measured using the analytical balance. Then a certain amount of magnesium sulfate was put into the crucible and the mass was measured again. Both the measurements were recorded. Then the crucible containing the magnesium sulfate was heated for 15 minutes. After the time was up, the flame was turned off and the crucible was removed from the clay triangle using crucible tongs. After cooling down, the mass of the anhydrate and the crucible was measured using the analytical balance. The final measurement was recorded. The anhydrate was disposed properly and the materials were cleaned and put back to their respective places.


Empty Crucible16.749g
Crucible with Hydrate Magnesium Sulfate18.293g
Hydrated Magnesium sulfate1.5440g
Crucible with anhydrate magnesium sulfate17.516g
Anhydrate magnesium sulfate0.77700g

Amount of water left –
Mass of hydrated MgSO4 – Mass of anyhdrate MgSO4
1.544g – 0.77700g = 0.7670g of H2O

1.Calculate the percentage of water in the hydrate. Explain why the size of the sample tested would not change this answer. Percent of water = (mass of H2O/ mass of hydrate) x 100
(0.7670g of H2O/ 1.544g of hydrate) x 100
= 49.68% H2O

2.Determine the formula of the hydrated salted.
a.Calculate the number of moles of water driven off from your sample. 0.7670g H2O x 1 mole H2O = 0.04261 moles H2O
18.001g H2O

b.Calculate the number of moles of anhydrous salt remaining in the crucible 0.77700g anhydrate MgSO4 x...
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