Lab: Determination of the Formula of a Hydrate

Chemical Quantities
Lab #3

LAB: DETERMINATION OF THE FORMULA OF A HYDRATE

OBJECTIVE: Given a sample of hydrated salt and appropriate apparatus, the student will be able to experimentally determine the correct formula for the compound. Remember that all salts are ionic compounds and as such have some attraction for water molecules. Even in a solid salt, water molecules can become trapped in the crystal lattice of the salt. When using a salt in a chemical reaction it can be important to know how much of the mass of the salt is due to trapped water molecules. When salts are used, their formula may be written to indicate the hydration factor: example KCl • 3H2O would be potassium chloride trihydrate indicating that there are 3 water molecules per KCl formula unit trapped in the crystals.

APPARATUS: goggles, Bunsen burner, matches, crucible, triangle, crucible tongs, ringstand and ring, spatula, balance.

PROCEDURE:

1. Weigh a crucible to the nearest 0.01 gram. Add approximately 2 grams of salt. Record total weight of salt and crucible to the nearest 0.01 gram. Subtract to find the weight of your salt.
2. Support the crucible on the triangle at an appropriate height to receive the maximum heat of the flame. WARNING: hydrates, upon heating, can decrepitate (explode). Protect your eyes at all times. Heat the crucible gently at first. Too rapid heating may cause loss by spattering because the water of hydration is driven off too rapidly. Gradually heat more strongly, keeping the crucible in the hottest part of the flame. After ten minutes of strong heating, gradually withdraw the flame, wait a moment, then use the crucible tongs to move the crucible to the base of the ringstand to cool for at least three minutes. Never put a hot crucible on a balance. When cooled enough to touch, move to the balance with crucible tongs, weigh and record.
3. Repeat the heating, cool, and reweigh. Continue this process until you get two successive weighing within 0.20 grams. This is called “heating to constant weight”. Discard the used, now anhydrous salt in the trash. The crucible may then be washed out with tap water.
4. SHOW ALL CALCULATIONS CLEARLY AND NEATLY. WATCH SIGNIFICANT DIGITS.
5. Use the lowest weight to find the mass of anhydrous salt.
6. Find the formula mass of your salt.
7. Find moles of your salt.
8. Subtract the two masses to find the mass of water
9. Find moles of water
10. Write the formula for the hydrate
11. Calculate the percentage of water in the hydrated salt
12. Find the formula mass of the hydrated salt.
13. Calculate the “theoretical percentage of water in the hydrate

1
HC/CC/TG KHS