Honors Chemistry

Hydrate Lab

Purpose:

In this experiment, you will study some of the properties and characteristics of several ionic hydrates and learn their naming and formula writing systems.

Background:

Chemical compounds that contain a discrete amount of water molecules as part of their crystalline structure are called hydrates. Hydrates occur quite commonly among chemical substances, especially among ionic substances. Such compounds are often prepared in, or are re-crystallized from, aqueous solutions.

Hydrates are most commonly encountered in the study of metal salts, especially salts of the transition metals. Water is bound in most hydrates in definite stoichiometric proportions, and the number of water molecules bound per metal ion is often characteristic of a particular metal ion.

A very common hydrate often encountered in the lab is copper(II) sulfate pentahydrate, CuSO4.5H2O. The word “pentahydrate” in the name of this substance indicates that five water molecules are bound in this substance per copper sulfate formula unit. Hydrated water molecules are generally indicated in formulas by using a dot to separate the water molecules from the formula of the salt itself. The naming of ionic hydrates includes the ionic portion being named based on the rules for naming ionic compounds, the number of water molecules is indicated with a molecular prefix, and followed with the word hydrate.

Many hydrated salts can be transformed to the anhydrous (without water) salt with heat. For example, when a sample of copper (II) sulfate pentahydrate is heated, the bright blue crystals of the hydrate are converted to the white, powdery, anhydrous salt.

CuSO4.5H2O (s) CuSO4(s) + 5H2O(g)

During the heating of copper (II) sulfate pentahydrate, the water of crystallization is clearly seen escaping as steam from the crystals.

It is also possible to reconstitute the hydrate by adding water to the anhydrous salt. The solid will reassume the color of the hydrated salt. Anhydrous salts are sometimes used as chemical drying agents, or desiccants, because of their ability to combine with water from their surroundings. For example, most electronic equipment, shoes, etc. come packed with small envelopes of desiccant to protect them from moisture. Substances that absorb moisture and are able to be used as desiccants are said to be hygroscopic.

Lastly, sometimes hydrates decompose when heated and form a metal oxide rather than simply releasing their water. This is important because you should not assume that heating will always result in the release of water and a residue of ionic solid.

Safety:

  1. goggles and aprons
  2. copper, cobalt, nickel, chromium, and barium compounds are highly toxic. Wash hands after use and always use spatulas to transfer chemicals.
  3. When heating the samples they may spatter out of the crucible. To avoid spattering heat the sample slowly with a low flame and more vigorously after the color change has occurred to assure completion.

Procedure:

  1. Mass a clean dry crucible.
  2. Add a spatula tip full (about the size of a pea) of one of the hydrates to the crucible and mass.
  3. Record observations of the crystals before heating.
  4. Place the crucible in the clay triangle and begin heating with a low flame.
  5. IF YOU SEE YOUR SAMPLE GETTING TOO HOT (SPATTERING OR ABOUT TO) SLIDE THE BURNER OUT FROM UNDERNEATH AND THEN LOWER THE FLAME AND CONTINUE
  6. Record observations once you see the apparent color change.
  7. When the sample is nearly dehydratedheat another minute to assure the dehydration goes to completion.
  8. Turn off the burner and allow the crucible to cool for 5 minutes.
  9. Mass the cooled crucible.
  10. Add one or two drops of water from the de-ionized water bottle to the crucible and record observations.
  11. Clean out the crucible by running it under water to dissolve the hydrate and flush it down the sink with plenty of water.
  12. Dry the crucible with paper towels.
  13. Heat the crucible, empty, for 1 minute to dryness.
  14. Cool the crucible.
  15. Obtain a different crucible and repeat the procedure for the next sample.
  16. After washing the second crucible use the first crucible, which should now be cool to analyze your 3rd sample.
  17. The crucibles should be washed and paper towel dried after samples 2 and 3.

Pre Lab QuestionsName ______

1) What is the objective of this lab? ______

______

______

2) Define the following:

Hydrate:

Anhydrous:

Hygroscopic:

3) How is the water removed from the hydrated crystal? ______

______

4) Do hydrates always lose water when heated? Explain!

______

______

______

5) Name or write formulas for the following hydrates. Remember, the prefixes for the number of hydrated crystals are the same as those used for naming molecular compounds.

Pb(NO3)2. 4H2O
Zinc chromate trihydrate
Cr3(PO4)2. 7H2O
Iron (III) sulfide decahydrate
NiCl2.6H2O
Cobalt (I) dichromate dihydrate

Data and Observations:

Hydrated salt / Copper(II) sulfate / Nickel(II) sulfate / Cobalt(II) chloride
Mass of empty crucible (g)
Mass of crucible and sample (g)
Observations before heating
Mass of crucible and sample after heating (g)
Observations after heating
Observations after addition of water
Theoretical chemical formula for each hydrate

Lab Report:

  1. prelab questions and data table page, completed
  2. procedure – include a heading for this section and a single sentence referring to the procedure in the lab handout
  3. calculations – neatly write or type this section including a heading for the section, a heading for each calculation, the words describing the calculation, the math for the calculation, and the final labeled answer.
  4. Mass of hydrate
  5. Mass of anhydrous residue
  6. Mass of water removed
  7. Experimental % of water in the hydrate
  8. Correct (theoretical) % of water in the hydrate(based on the correct chemical formulas)
  9. Percent error based on iv and v above
  1. conclusion – a paragraph that includes the following: a response to your purpose, a statement of your final result, human errors, and errors beyond your control. (there are no additional questions with the lab)