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Homework Help: Experiment on water of crystallization

  1. Sep 28, 2012 #1
    1. The problem statement, all variables and given/known data
    Aim- To determine the percentage content of water (H2O) in hydrated copper(II) sulphate crystals (CuSO4 . 5H2O).
    Equipment- Approximately 1 gram of copper sulphate crystals, evaporating dish, tongs, Bunsen burner, gauze, tripod, spatula, weighing balance.
    Method- a) Set up the Bunsen burner, gauze and tripod.
    c) Weigh the evaporating dish and approximately 1 gram of copper sulphate crystals.
    d) Gently heat the copper sulphate crystals in the evaporating dish, until they appear to be white (If heated too much, the crystals will break down further and turn yellow.)
    e) Take the evaporating dish away from the heat with the tongs, and weigh the evaporating dish containing the now white crystals.
    f) To determine the percentage water, you subtract the weight of the crucible from the final weight, and then you subtract the weight of the substance left from the initial weight of the copper sulphate crystals.

    2. Relevant equations

    3. The attempt at a solution
    Please see the bold words.I wonder why yellow solid is formed.I have no idea coz I don't think any yellow substance will be formed among Cu,S,O and H.At first I thought it is because sulphur is formed but CuO is black in colour and Cu(OH)2 is pale blue and so they would not give a yellow appearance.Can anyone help me?Thx.
  2. jcsd
  3. Oct 2, 2012 #2
    If you really do want to follow up on this one, the first thing you really need to do is to discuss it with your instructor.
    There is absolutely no point in speculating about what might be causing a yellow colour unless and until you can repeat the experiment, and slightly overcook the crystals, and observe that you can, in fact, produce yellowish material.

    If you do have to account for a yellow material, then sulfur does not get a guernsey. Check out the following possible products from the thermal decomposition of anhydrous copper sulfate. It is pretty unlikely that anything containing hydrogen will be left. But other possible products include

    Basic copper sulfates, like CuO.CuSO4
    Copper oxide -- usually black, but can sometimes appear brown.
    Sulfites of copper (oxygen loss)
    Copper(I) compounds -- sulfate, sulfite, or oxide.

    The usual thermal decomposition path for a metal sulfate is (1) dehydration to lose water co-ordinated to the metal (2) dehydration of the last water of hydration, normally associated with the sulfate (3) deoxygenation sulfate to sulfite (4) loss of sulfur dioxide for sulfite to oxide. The order of the first two steps is sometimes reversed, and the last two steps sometimes occur together. For copper, the situation is a little complicated, because copper(I) compounds, which do not participate much in the aqueous chemistry of copper, are much more prominent in the dry, and at high temperatures.

    Finally, remember that it is not always the case in chemistry that a simple mixture of two substances will have the colour you might expect, especially if the particles are colloidal size or less.
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