MHS Chemistry
Determination of the Formula of a Hydrate

The elements hydrogen and carbon combine to form thousands of hydrocarbons.  The formulas of these compounds range from CH4 to C7H12 to C14H10.  But: you will never find a compound with the formula C½H¾.  In fact, when Dalton formulated his atomic theory in the early 1800s, he indicated that (1) only whole atoms, not fractions of atoms, react with one another, and (2) when atoms of two or more elements combine to form more than one compound, they may combine in different proportions, but always in ratios of small whole numbers.

The simplest whole number ratio of elements in a compound is called the empirical formula. In contrast, the molecular formula of a compound is the actual number of atoms of each element in each molecule of the compound.  In this experiment, you will determine the empirical formula of a compound of unknown composition.

Objectives

1. Determine the relative number of moles of barium, chlorine, and water in the compound barium chloride hydrate.
2. Calculate the lowest whole number ratio of moles of barium, chlorine, and water in barium chloride hydrate.
3. Determine the empirical formula of barium chloride hydrate.
Procedure
1. Determine the mass of an empty crucible or evaporating dish. Record the mass to the nearest 0.001 g.
2. Add between 0.500 and 1.000 g of barium chloride hydrate to the crucible.
3. Determine the mass of the crucible and salt. Record the mass.
4. Place the crucible on a clay triangle, set up as shown by your instructor.  Begin heating the sample slowly.
5. After about 10 minutes of gentle heating, increase the heat until no further change in the salt is apparent.  At this point, the salt will seem to have changed from a crystalline solid to a grainy solid.
6. Use tongs to place the crucible on a different clay triangle to cool.  Make sure the tongs do not have rubber on them.
7. When the crucible has cooled sufficiently to be handled, again determine the mass of the crucible and contents.
8. Return the crucible to the first clay triangle and re-heat it.  Allow it to cool, then determine the mass of the crucible' with its contents again.
9. Continue this process until a constant mass is attained.
10. Record the lowest mass of the crucible and anhydrous salt.
11. Transfer the anhydrous salt to a 250 mL beaker.
12. Dissolve the anhydrous salt in a minimum (about 25 mL) of distilled water.
13. Add about 20 mL of 1.0 M silver nitrate solution to the beaker.  (Caution: silver nitrate will burn the skin and leave brown stains that cannot be washed off for about a week).
14. Stir the contents of the beaker thoroughly.  The precipitate is silver (I) chloride.
15. Place the beaker on the ring stand and heat the contents to boiling.
16. While the contents are heating, determine and record the mass of a single sheet of filter paper.
17. Set up a funnel and beaker as demonstrated.
18. Use the tongs to remove the beaker from the heat and filter the precipitate from the solution.
19. Allow the filter paper to dry overnight.
20. Determine and record the mass of the dried filter paper with the precipitate.
21. Give the filter paper and dried precipitate to your teacher for disposal.

Data
 label 1. crucible and cover _______________ _____ 3. crucible, cover, and hydrated salt _______________ _____ 10. crucible, cover, and anhydrous salt (lowest) _______________ _____ 16. filter paper _______________ _____ 20. filter paper and precipitate _______________ _____

Analysis
 label 1. mass of hydrated salt _______________ _____ 2. mass of anhydrous salt _______________ _____ 3. mass of water lost _______________ _____ 4. moles of water in original sample _______________ _____ 5. mass of silver (I) chloride precipitated _______________ _____ 6. moles of silver (I) chloride _______________ _____ 7. moles of chloride ion present _______________ _____ 8. mass of chloride ion present _______________ _____ 9. mass of barium in original sample _______________ _____ 10. moles of barium in original sample _______________ _____

Conclusions
1. Relative actual number of moles for hydrated salt:

Ba: ________   Cl: ________   H20: ________

2. Lowest whole number ratio for hydrated salt:

Ba: ________   Cl: ________   H20: ________:

3. Empirical formula for hydrated salt:

 Ba Cl •_______ H2O _______ _______

4. What is the proper name for this compound?

5. The formula mass of this compound is _______________. (Show your work.)

6. There were __________ moles of this compound present when you started. (Show your work.)