Chemical Formula Hydrate Lab

  1. We did a lab at our school recently but some of the questions regarding the lab confused me.
    The data obtained from the lab is here:
    Determining the Chemical Formula of a Hydrate

    Purpose
    Find the molecular formula of the hydrate of Copper (II) Sulfate, CuSO4 x H20.

    Prediction
    When the solution is heated the hydrate will convert to an anhydrous ionic compound. Then the percentage composition will be determined by weighing the mass of the white crystals.

    Materials
    • 400ml beaker with hot plate
    • Tongs
    • Scoopula
    • Electronic balance
    • Glass rod
    • 3g to 5g hydrated Copper(II) Sulfate
    • Goggles

    Procedure
    This experiment was done in order to find the percentage composition of a hydrate compound. The mass of the empty beaker, beaker + hydrated Copper (II) Sulfate, and mass of the beaker + anhydrous Copper (II) Sulfate was found through this procedure.
    The mass of a beaker and stirring rod was found. About 4g of hydrated Copper (II) Sulfate was added to the beaker and the weight was recorded. The beaker was placed on a hot plate, heated until the blue crystals lost there blue color. Stirring the compound was essential to making sure all the blue crystals were heated. When the crystals lost their color the heat source was removed and the beaker was left to cool. The mass of the white crystals, when cooled was recorded. The experiment compound was safely disposed of and the materials that were used were properly put away.

    Table 1: Masses
    Mass of Beaker and Stirring Rod 77.774g
    Mass of Beaker + Hydrated Copper(II) Sulfate 81.531g
    Mass of White Crystals 80.348g






    Mass of CuSO4 x H2O Mass of H2O= 18.0152g Mass of CuSO4= 159.6068g
    2(1.00794)
    1(15.9994) Mass of Hydrated CuSO4= 3.757g Mass of Anhydrous= 2.574g
    1(63.546)
    1(32.065)
    4(15.9994)
    177.62g

    Analysis Questions Page 227 in Text

    1a.) 3.757 Difference 1.183 / 3.757 x 100% = 31.49%
    -2.574
    1.183
    b.) It is expected that the mass percent of water should be similar in all the groups,
    concerning the Hydrated CuSO4 x H2O.

    2a.) Table 2: Class Group Results
    Group Mass(CuSO4 x H2O) Mass(CuSO4) % H2O
    1 3.757g 2.574g 31.49%
    2 3.587g 2.277g 36.52%
    3 3.871g 2.646g 31.65%
    4 3.188g 2.075g 34.91%
    5 3.465g 2.168g 37.43%
    b.)

    3.) CuSO4= 100 - 31.49 H2O Class Average= 34.4%
    = 65.6g

    34.4g (1 mol/18.01528)
    =1.909mols

    65.6g (1 mol/ 159.6086) Therefore the Molecular Formula is CuSO4 x 5H2O.
    =0.41mols

    1.909/ 0.41= 4.66
    = 5 H2O molecules
    4.) If you were to heat the hydrated ionic compound in a test tube you would expect to
    see steam rising which means that the water in the compound is being evaporated into
    the air. In our experiment we did see this happen but it would be more evident in a test
    tube because it is more confined than the beaker.


    Here are my questions that I'm very confused with, your help is much appericiated:

    1. You obtained the mass percent of water in the copper sulfate hydrate.
    a.) Using your obervations, calculate the percentage composition of the copper sulfate hydrate.
    b.) In the case of a hydrate, and assuming you know the formula of the associated anhydrous ionic compound, do you think it is more useful to have the mass perventage composition? Explain your answer.

    2. Suppose that you did not compelely convert the hydrate to the anhydrous compound. Explain how this would affect the calculated percent by mass of water in the compound. and the molecular formula you determined.

    3. Suppose the hydrate heated too quickly and some of it spatted out of the container and was lost. Explain how this would affect the calculated percent by mass of water in the compound and the molecular formula you determined.
    (I understand that it would change, but exactuly how?)

    4. Suggest a source of error (not already mentioned) that would result in a value of x that is
    a.) higher than the actual value
    b.) lower than the actual value

    I know this quite a few questions...but i really dont understand. Please help:bugeye:, thank you :)
     
    Last edited: Apr 12, 2007
  2. jcsd
  3. chemisttree

    chemisttree 3,723
    Science Advisor
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    What don't you understand?
     
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