1 · Heat to constant mass
Heat copper(II) sulfate with the Bunsen flame. Record n after every heating. When does it stop changing, and what is n after only one?
AP Chemistry · Topic 1.3 · Elemental composition of pure substances
Heat a hydrate in a crucible, weigh it after each heating, and find how many waters each formula unit holds. Then find out what each lab mistake does to the answer, and name unknown hydrates.
| # | Hydrate | Heat or mistake | Heatings | Last reading (g) | n |
|---|
Record adds the current run. Copy puts the table on the clipboard for a spreadsheet.
Heat copper(II) sulfate with the Bunsen flame. Record n after every heating. When does it stop changing, and what is n after only one?
Heat it on the hot plate until the mass stops changing. Is n 5? Then try barium chloride on the hot plate. Why the difference?
Heat copper(II) sulfate very strongly. What color is the powder, what happens to the mass, and to n?
Predict, then run: spattering, and letting the powder cool in open air. Why do they push n in opposite directions?
About the model. The balance reads to 0.001 g and the crucible is cooled before each weighing (except in the “weighed hot” mistake). In the model, the loosely held waters leave at a steady rate set by the heat, the tightly held ones (copper(II) sulfate's last water, cobalt chloride's last two, gypsum's last half) leave more slowly and only with a flame, and the last traces die away gradually. The rates were chosen so the standard copper(II) sulfate run (2.500 g, Bunsen flame, 5 minutes a heating) gives the Topic 1.3 deck's example readings: 23.212, 22.996, 22.951 and 22.950 g. Very strong heat breaks copper(II) sulfate down to black CuO and SO₃ at a rate chosen to match the deck's overheated reading. Real rates depend on the flame, the crucible and the size of the crystals; temperatures here are approximate, and the other salts are treated as stable at these temperatures.
Sample masses are the deck's (Epsom salt 2.465 g, cobalt chloride 1.190 g, washing soda 2.862 g, gypsum 1.722 g); molar masses use the AP periodic table, with H₂O at 18.02 g/mol, as on the slides.
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