Compound Stoichiometry - Mole Concept Consider a sample of copper(II) sulfate pentahydrate. It's a blue coarse crystalline substance and is sitting on a laboratory balance as shown below. The chemical formula for copper(II) sulfate is CuSO 4 ·5H 2 O. Note that the “·5” in front of the H 2 O means that 5 waters of hydration are part of the formula. The “·5” does NOT mean times ﬁve ( × 5). If the blue hydrated copper(II) sulfate is heated to 110˚C, 4 of the 5 hydrates (water molecules) will come off leaving only 1 hydrate. The formula for copper(II) sulfate monohydrate would be CuSO 4 ·H 2 O.and it is a very pale blue color. Note how there is only one hydrate left. It is “held” tighter than the other 4 hydrates and will not come off at 110˚C. However, if it is heated to 150˚C (or higher) the last hydrate comes off and you would then have anhydrous copper(II) sulfate which has a chemical formula of CuSO 4 and is white in color. Needless to say, the weights of equal amounts (moles) of each of these compounds will be different because of the waters of hydration.
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