Principles of Chemistry: Lab Online



Majority of salts in their crystalline state associate with water.  Such salts are identified as hydrates. The bonds holding the molecules of water to the salt are as a result of the salt’s ionic nature and the polarity of water. (Weiner S.A., & Harrison B., 2010). These bonds however, are weakly attached. Simple heating therefore can drive off the water leaving an anhydrous salt substance. The mass of water forms part of the hydrate mass. This means therefore that after heating the mass of the hydrate changes. 


On heating a hydrate, the mass of the compound changes. The compound will have a lower molar mass hence the number of water molecules in each unit of the anhydrous salt can be determined. ( Kenkel, 2010).

Apparatus and materials

Evaporating dish

Bunsen burner

Wire gauze

A balance

Stop watch

Hydrous Magnesium sulfate

Materials and Methods

After recording the name and formula of the unknown hydrate, an evaporating dish was placed on the center of the wire gauze atop a stand. A Bunsen burner was used to heat the dish and drive off any impurities such as oil or water that may have collected on it.

After about 2 minutes of the initial heating, the evaporating dish was masses using a balance.

The evaporating dish was placed on the table and about two spoons of the unknown were placed on it.

The mass of the evaporating dish with the unknown was measured and recorded using a balance.The evaporating dish and the unknown were returned to the stand and heated for 3 minutes at a low flame.

The evaporating dish was kept in place and heated for another 5 minutes on high, after which it was removed and allowed to cool before measuring its mass on the balance. This mass was recorded

The evaporating dish and anhydrous salt were returned to the stand and heated for another 2 minutes. The mass was determined again with the balance. This action was repeated until 2 successive masses were the same. This was done to ensure that all of the water had been removed from the compound. This value was recorded as mass of evaporating dish and anhydrous salt.


Water in a Hydrate Data Table
Unknown NameMagnesium sulfate
Unknown FormulaMgSO4 * ?H2O
Mass of empty evaporating dish168.386 g
Mass of dish with unknown181.368 g
Mass of unknown compound12.982 g
Mass of dish with anhydrous salt174.725 g
Mass of anhydrous salt6.339 g
Mass of water lost6.643 g
Moles of anhydrous salt0.05266 mol
Moles of water0.3691 mol
Number of water molecules per formula7 mol


Mass of unknown: 181.386 – 168.386 = 12.982 g

Mass of water lost: 181.386 – 174.725 = 6.643 g

Mass of anhydrous salt: 12.982 – 6.643 g = 6.339 g

Moles of water: 6.643 g (1mol/18g H2O) = 0.3691mol

Moles of anhydrous salt: 6.339 g (1mol/120.38g) = 0.05266mol

Molar ratio between water and anhydrate: (0.3691/0.05266) = 7:1

Empirical formula of the salt hydrate: MgSO4 .7H2O

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Discussion and Conclusion

The three potential sources of error are:

Not heating the evaporating dish before adding the sample

If this step is skipped, any water in the evaporating dish prior to adding the sample will be accounted as being part of the compound. This will give results of a higher mass of water being lost, which be will analyzed as a greater number of water molecules per formula unit.

Starting to heat the sample on high flame instead of gradually

This could cause the compound to foam and spill over the evaporating dish. If the lost mass is not accounted for the calculations of molar mass will be messed up.

Taking the mass of the anhydrous salt too soon before having two consecutive masses being the same

This will yield a lower number of water molecules per formula unit.

The dehydration and re-hydration is a reversible process. (Ludwig, 2000).Therefore it is possible to re-hydrate whereby water can be added to the anhydrous salt to make a solution. Then the solution would be allowed evaporate the excess water.

The data collected in this lab supports that water molecules are loosely bound to a salt under normal conditions. When these conditions were changed, like applying direct heat to the salt, water evaporated leaving an anhydrous salt behind. The molar mass ratio of the lost water and anhydrous salt was calculated using stoichiometry. This ratio was the number of moles of water molecules per formula unit. With all of this data the empirical formula of the hydrate compound was found.

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  1. Weiner S.A., & Harrison B. (January, 2009). Introduction to Chemical Principles: A Laboratory Approach. Saratoga: Cengage Learning.
  2. Kenkel J. (2010). Analytical Chemistry for Technicians, Third Edition. (Pg. 57) United States: CRC Press.
  3. Ludwig L. V. (2000). Inquiry-based Experiments in Chemistry. (Pg. 197). United Kingdom: Oxford University Press.
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