Formula of a Hydrate



The purpose of this experiment is to determine the empirical formula of a hydrate. Hydrates are inorganic salts which contain a specific number of water molecules loosly attached. Examples are: magnesium sulfate heptahydrate (epsom salts) and sodium carbonate decahydrate (washing soda). The formulas for these substances are MgSO4*7H2O and Na2CO3*10H2O. They can also be represented as MgSO4(H2O)7 and Na2CO3(H2O)10. Not all hydrates have simple formulas like these. For example, a hydrate of cadmium sulfate seems to have 2.66 molecules of water for each molecule of CdSO4. This hydrate is best represented as (CdSO4)3(H2O)8. When determining theformula of a hydrate you must not assume that it is one with a simple formula.

Hydrates can normally be decomposed into the anhydrous salt and water by gentle heating. From the data collected the number of molecules of hydrated water will be determined per molecule of anhydrous salt.

Be sure to adhere to the safety precaustions given by your instructor.


Sample Data and Calculations

Hydrate used: (CuWO4)x(H2O)y

mass of empty crucible: 18.546 G

mass of curcible plus hydrate: 20.832 G

mass of curcible + contents (1st. heating): 20.597 G

mass of curcible + contents (2nd. heating): 20.596 G

mass of hydrate used: 20.832 - 18.546 = 2.286 G

mass of water driven off: 20.832 - 20.596 = 0.236 G

mass of anhydrous salt left behind: 20.596 - 18.546 = 2.050 G

percentage of water in hydrate: 0.236 / 2.286 = 10.3 % ( 3 significant digits allowed )

percentage of anhydrous salt left behind 2.050 / 2.286 = 89.68 % ( 4 significant digits allowed)

mols of anydrous salt present in hydrate = 2.050 / 311.392 = 0.006583

mols of water present in hydrate = 0.236 / 18.01 = 0.01310

mol ratio of water to salt 1.99 to 1 (2 to 1)

Formula CuWO4(H2O)2

Mass a crucible and its cover and record in a data table. Place about 2 grams of the hydrate in the crucible and record the mass of the crucible, cover and hydrated salt. Gently heat the crucible and its contents for about 10 minutes. During the last minute of heating remove the cover so that any moisture which has collected on the underside of the cover can evaporate. When the crucible has cooled determine the mass of the crucible, cover, and contents.

Replace the cover and heat the crucible for five more minutes removing the cover during the last minute of heating as you did above. Cool and determine the mass of the crucible, cover, and contents. This last mass should agree with the previous mass to within plus or minus 0.005 grams. If it does not repeat this heating until a constant mass is reached. This is called heating to a constant mass and is the only way of insuring that the reaction is complete.


(All answers must be reported to the proper number of significant digits.)

1. Determine the mass of water driven from the hydrate during the heating process.

2. Determine the mass of anhydrous salt left in the crucible after the heating process.

3. Determine the percentage of water in the hydrate.

4. Determine the empirical formula of the hydrated salt.

[Teacher Notes]

There are several hydrates that can be used for this lab. I have used the following over the years. magnesium sulfate heptahydrate, copper sulfate pentahydrate, sodium carbonate monohydrate, and sodium carbonate decahydrate. The last one decomposes so easily, that an open container of it will dehydrate. An interesting variation on this lab is to have students calculate the degree of decomposition of sodium carbonate decahydrate in an old sample of the salt.  

Questions? Comments??