Finding the Molecular Formula of an Unknown Hydrate
The purpose of this lab was to find the amount of water and, thus, the molecular formula of an unknown hydrate. We accomplished this by massing the hydrate, boiling off the water and measuring the resulting anhydrous compound. We indirectly calculated the mass of the water and then used these values to derive the molecular formula.
1. Adjust the Bunsen burner so that it produces a flame with a blue (oxidizing) cone about 2-3 cm above its top. Take care to adjust correctly; this is crucial to success. Consult your instructor if you have difficulty adjusting the burner.
2. Weigh the container and glass cover to be used in the experiment; then add 1.5 g of hydrate to the container and weigh again.
3. Place the glass cover on the container and heat the container for 4-8 minutes (the container should be about 5.0 cm above the flame’s oxidizing cone).
4. Once the compound appears blue and dry, use tongs to remove the container (do NOT touch it with your hands) and place it in the desicooler to cool to room temperature. For efficiency, clean and heat a crucible for the second sample while the first is cooling. After the container has cooled, weigh it.
5. Repeat the trial for a second sample.
6. Calculate the formula of the hydrate.
7. If the two samples do not agree within 2% or if either differs from the correct value by over 4%, consult your instructor before continuing. Since the procedure was probably not followed perfectly, you will probably be asked to repeat the trial.
Before Heating: Burgundy and Rock salt-like/Crystal-like
After Commencement: Looks moist; bubbles and clings together. Starts to spatter onto cover; pops every once in a while; starts to cluster together and becomes foamy and blotchy. Color now light blue. Water vapor becomes very obvious and starts to gather on cover.
The End: Simmers down, turns completely blue, dries out and becomes sand-like. Vapor evaporates.
IV. Equations and Chemical Formulas
Mass of Container = (Mass of Container + Mass of Hydrate) – Mass of Hydrate
Mass of Water Eliminated = (Mass of Container + Mass of Hydrate) – (Mass of Container + Mass of Anhydrous Compound)
Molar Mass of Water = 18 g; Molar Mass of Anhydrous Compound = 129.9 g
Moles of Water Eliminated = (Mass of Water Eliminated) / (Molar Mass of Water)
Mass of Anhydrous Compound = (Mass of Container + Mass of Hydrate) – (Mass of Container)
Moles of Anhydrous Compound = Mass of Anhydrous Compound / Molar Mass of Anhydrous Compound
Formula of Hydrate = Moles of Water Eliminated / Moles of Anhydrous Compound = Ratio of Water to Anhydrous Compound
Average Formula = ½ * (Formula of Hydrate in Sample 1 + Formula of Hydrate in Sample 2)
Percent by Mass of Water in Unknown = Mass of Water / (Mass of Water + Mass of Anhydrous Compound)
Average Percent by Mass of Water = ½ * (Percent by Mass of Water in Unknown in Sample 1 + Percent by Mass of Water in Unknown in Sample 2)
V. Data and Calculations
Mass of Container + Hydrate (Measured)
Sample 1 51.63 g
Sample 2 37.47 g
Mass of Container (Formula)
Sample 1: 51.63 g – 1.5 g 50.130 g
Sample 2: 37.47 g – 1.502 g 35.908 g
Mass of Hydrate (Measured)
Sample 1 1.500 g
Sample 2 1.502 g
Mass of Container + Anhydrous Compound (Measured)
Sample 1 50.925 g
Sample 2 36.7 g
Mass of Water Eliminated (Formula)
Sample 1: 51.63 g – 50.925 g 0.705 g
Sample 2: 37.47 g – 36.676 g 0.708 g
Moles of Water Eliminated (Formula)
Sample 1: 0.705 g / 18 g 0.03917 mol
Sample 2: 0.708 g / 18 g 0.03933 mol
Mass of Anhydrous Compound (Formula)
Sample 1: 50.925 g – 50.13 g 0.795 g
Sample 2: 36.700 g – 35.97 g 0.792 g
Moles of Anhydrous Compound (Formula)
Sample 1: 0.795 g / 129.9 g 0.00612 mol
Sample 2: 0.730 g / 129.9 g 0.00610 mol
Formula of Hydrate (Formula)
Sample 1: 0.03917 mol / 0.00612 mol = 6.4/1 CoCl2 • 6H2O
Sample 2: 0.03933 mol / 0.00610 mol = 6.4/1 CoCl2 • 6H2O
Average Formula: (Formula) ½ (6.4 + 6.4) = 6.4 CoCl2 • 6H2O
Percent by Mass of Water in Unknown (Formula)
Sample 1: 0.705 / 1.5 47%
Sample 2: 0.708 / 1.502 47.1%
Average Percent by Mass of Water: (Formula) ½ (47 + 47.1) 47%
The purpose of this lab was to find the formula and composition of a hydrate. We accomplished this by finding the weight of the substance before and after the water was eliminated from it. At first, we had some fear in the lab that we weren’t doing it correctly, as we didn’t have a chaperone, but all was well. We received tangible results and, upon checking with our instructor, Mr. Mylin, learned that they were correct. This lab was a success. We gained knowledge and experience in both chemistry and teamwork.