Determining the Chemical Formula of a Hydrate

Purpose: Find the molecular formula of the hydrate of Copper (II) Sulfate,

CuSO4 x _?_H20. You need to find the number before the H2O. Or put another way…

How many moles of water are there for every one mole of CuSO4?

Background: When the chemical is heated the hydrate will convert to an anhydrous ionic compound (this means the water will leave it). The moles of H2O will be determined by assuming the amount of mass lost by heating is the mass of the evaporated H2O.

The moles of the CuSO4 (the white substance after heating) can be calculated from the mass of the white crystals. Using the molar ratio of moles of CuSO4 to moles of

H2O, one can determine the chemical formula of the hydrate.

Procedure 1) Mass everything. (done before heating) TRIAL 1 TRIAL 2 a) Mass of the empty beaker ______

b) Mass of hydrated CuSO4 (blue) ______

c) Mass of beaker + hydrated CuSO4 ______

2) Place the beaker on a hot plate, heat (on about a setting 5-7) until all the blue crystals lost their blue color. Stir to make sure all the blue crystals are heated.

3) Remove the beaker from heat and allow it to cool down. Record the mass of the new white crystals. TRIAL 1 TRIAL 2

d) mass of the beaker + anhydrous CuSO4 (white) ______

4) Determine the mass of H2O in the hydrated sample(c - d) ______

5) Convert the mass of water to moles of water ______

6) Determine mass of anhydrous CuSO4 (d - a) ______

7) Convert the mass of CuSO4 to moles of CuSO4 ______

8) Find the simplest whole number ratio of moles of CuSO4 to moles of water (Hint:divide by lowest number of moles) TRIAL 1 TRIAL 2 The formula for the hydrate is: ______

9) Pour the cooled white CuSO4 in the waste container. Clean and dry your beaker, then Repeat steps 1-8 one more time for trial 2. If you answers differ greatly, do a third trial.

10) Ask Dr. Bryan for the correct answer and Calculate your % error TRIAL 1 TRIAL 2

______

11) Clean up. Then answer the following questions. You will need to answer these and add them to your lab report.

1. You obtained the mass percent of water in the copper sulfate hydrate. a.) Using your observations, calculate the percentage composition of the copper sulfate hydrate. % comp = (mass of part/mass of whole)

2. Suppose that you did not completely 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 splattered 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 exactly how?)

4. Suggest a source of error (not already mentioned) that would result in a value of the moles of water (the ? …in CuSO4 + _?_ H2O) that is… a.) higher than the actual value b.) lower than the actual value