1. To determine the percentage of water in an unknown hydrate.
2. To determine the moles of water present in each mole of this unknown hydrate when given the molar mass of the anhydrous salt.
3. To write an empirical formula of the hydrate.
Supplies
Equipment: lab burner, crucible and lid, crucible tongs, pipe stem triangle, ring stand and ring, centigram or digital balance, lab apron, safety goggles.
Chemical reagents: approximately 5 g of a hydrate, water.
Procedures
1. set up equipments, make sure flame is blue
2. heat empty crucible
3. cool empty crucible
4. measure the mass of crucible
5. fill 1/3 of crucible with hydrate, record mass
6. heat and then cool, record mass
7. reheat & cool, record mass
8. take the lower mass in 6 and 7
9. add water to the content, record changes.
Questions That Might Help You to Understand the Lab Better
1. What did you learn from the lab?
A: 1) we can decide some hydrates' percent composition by heating
2) we cannot decide some hydrates' percent composition because they would decompose before heating is done.
3) Hydrates might have different colors than their anhydrous salts.
4) Not all hydrates would go through a color change during heating.
2.The reason the answer is not accurate:
A: 1) the hydrate decomposes under high temperature (mass of anhydrous salt ↓)
2) the flame is not blue so carbon would accumulate under the crucible (mass of anhydrous salt ↑)
3) experiment errors
4)the crucible does not cool completely before weighing (mass of anhydrous salt ↓)
5) water is not completely driven off (mass of anhydrous salt ↑)
3. How to name the hydrates?
A: Na2CO3 10H2O = sodium carbonate decahydrate
CuSO4 5H2O = copper sulphate pentahydrate
(1 mono, 2 di, 3 tri, 4 tetra, 5 penta, 6 hexa, 7 hepta, 8 octa, 9 nona, 10 deca, 11 hendeka, 12 dodeca)
4. Why heating the crucible first before starting the experiment?
A: to drive off water in the crucible so the experiment would be more accurate.
