Hydrate LabName______

Date ______Period ______

Introduction:

Many compounds are formed as a result of reactions that occur in water solutions. These compounds appear to be dry, but when they are heated, large amounts of water are released. The water molecules are part of a crystalline structure and are weakly bonded to the ions or molecules that make up a compound. Such compounds are called hydrates, meaning that they contain water. The solid that remains when the water is removed is referred to as the anhydrous salt, or anhydrate.

Hydrate + Heat → Anhydrate + H20

Usually the amount of water present in the a hydrate is in a whole number ratio to the moles of the anhydrate. An example of a hydrate is magnesium sulfate. Its formula is MgSO4 ∙ 7H2O, indicating that seven moles of water are combined with one mole of magnesium sulfate in the crystalline form.

In this investigation you will be given an unknown hydrate and asked to determine the percent of water in the compound. From this information, the molar ratio of water to anhydrous salt will be calculated. Finally, the identity of the hydrate will be determined.

Pre-Lab Discussion:

  1. What is a hydrate? An anhydrate? ______
  1. Why do you think it is necessary to heat the crucible before finding its mass? ______
  1. Why must the crucible be cooled before finding the mass? ______
  1. Why must the mass of the anhydrous salt be measured immediately on cooling? ______
  1. Why is it necessary to handle the crucible only with the crucible tongs immediately after heating? ______
  1. What is the molar mass of the hydrate magnesium sulfate heptahydrate described in the introduction? ______
  1. What is the molar mass of the anhydrous salt, magnesium sulfate? The total water? ______
  1. What is the percent water in MgSO4∙7H2O? ______

Materials:

goggles and apronclay triangleunknown hydratebalance

ring standiron ringcrucible with coverspatula

lab burnerstrikercrucible tongsdesiccator jar

Procedure:

  1. Set up Bunsen burner, clay triangle and ring stand. Gently heat crucible and cover for three minutes. Then, allow crucible and cover to cool for three minutes in a desiccator jar.
  2. Measure and record the mass of the crucible and cover. Record in the data table to .001g.
  3. Add about 2 g of your unknown hydrate to the crucible. Measure and record the mass of the crucible, cover, and hydrate to the nearest 0.001g.
  4. With the lid slightly tilted, gently heat the unknown hydrate in the crucible until there is no more popping or spattering, or about 5 minutes.
  5. Remove the cover using the tongs and examine the material in the crucible. If the edges of the solid appear to be turning brown, reduce the heat momentarily and then begin heating again at a slower rate. Heat for 5 more minutes.
  6. Remove the crucible and allow it to cool for three minutes in the desiccator jar. Immediately measure and record the mass of the crucible, cover, and anhydrous salt.
  7. Reheat crucible, cover, and contents for 5 minutes, cool in the desiccator jar for three minutes and measure mass again. The value should be within 0.02 g of the last recorded mass. If it is not, reheat and remeasure the mass until the last two measurements are within that range. Record the final mass.
  8. Dispose of your substance down the drain or in the trash and clean up your work area.

Observations: List the name of your unknown hydrate: ______

Data Table:

A. Mass of crucible and cover
B. Mass of crucible, cover, and hydrate
C. Initial mass of hydrate alone (B-A)
D. Mass of crucible, cover and anhydrate (after step 6)
E. Mass of crucible, cover, and anhydrate (after step 7)
F. Mass of H2O lost (B-E)
G. Mass of anhydrate alone (E-A)

Calculations: (Show all work. Don’t forget significant digits and units!)

  1. Calculate moles of H2O lost:
  1. Calculate moles of anhydrous salt that remained:
  1. Determine the smallest whole number ratio between moles of water and anhydrous salt:
  1. What is the formula of the sample based on the experimental data?
  1. What is the % H2O in your hydrate according to the experimental data?
  1. What is the actual formula for your hydrate? ______(ask teacher)
  1. What is the actual % H2O in the hydrate from the formula?
  1. Calculate your percent error:

Questions:

  1. Explain the effect the following errors would have on the value for the percent water in a hydrate.
  1. The hydrate is not heated long enough to drive off all the water.
  1. A damp crucible was used, and it was not dried before adding the hydrate.
  1. The crucible and contents were allowed to cool overnight before finding their mass.
  1. Predict what would happen if you added a few drops of distilled water to the anhydrous salt remaining at the end of this experiment.

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