Hydrated Crystals

INTRODUCTION:

Many compounds are formed in reactions that take place in water solutions. The water is then evaporated to obtain the crystalline compound. In some cases water molecules are weakly attracted to the ions of molecules that make up the compound and are retained within the crystal structure. Crystalline compounds that retain water during evaporation are referred to as being hydrated or said to contain water of hydration. The ratio of moles of water to moles of compound is a small whole number. The formula for the hydrated compound barium chloride is:

BaCl2 · 2H2O
The dot shows that for every mole of BaCl2 in the crystal, there are 2 moles of H2O.

Many hydrates can be transformed to the anhydrous compound when heated strongly. For example, barium chloride dihydrate can be converted into anhydrous barium chloride. This change can be followed visually, often by a change in color. It is also possible to reverse the above process. If water is added to the anhydrous barium chloride, a color back to its original hue is obtained indicating that the dihydrate is regenerated. The property of reversibility can be used to distinguish true hydrates from other compounds that produce water when heated.
The ratio of moles of H2O to moles of compound can be determined experimentally in most cases by heating to remove the water. The compound with the water removed is anhydrous. In this experiment you will determine the formula for hydrated copper (II) sulfate. The formula is determined by comparing the mass of the hydrated and anhydrous forms of the compound.


PURPOSE:

In this experiment you will:
  • Observe several hydrates before and after heating each sample.
  • Heat a specific amount of hydrated copper (II) sulfate to remove the water.
  • Find the experimental and theoretical percentages of water in the hydrate.
  • Predict an empirical formula for hydrated copper (II) sulfate.


EQUIPMENT:

Goggles and apron
Hot plate
Bunsen Burner
Test tubes and test tube rack
Evaporating dish
600 mL beaker
Forceps
Electronic balance



INSTRUCTIONS:

  1. Each person in your lab group must read every page in this online procedure. Along the way, there will be questions that you must answer.
  2. Designate one person in your group as the Data Recorder. This person should open the Google document Experiment 7 - Hydrated Crystals for the approved answer template.
  3. When this Google document opens, sign in to your Google account.
  4. From the FILE Menu, choose Make a copy...
  5. From the FILE Menu, choose Rename...and rename the document as follows: Exp 7 - Period (1, 3, 6, or 8) - Group #.
  6. Share this document with the members of your group and with Mr. Skubis at HSTChemistry@gmail.com.
  7. As a group, answer all questions. Remember to use complete sentences and be mindful of grammar, spelling, and punctuation.
  8. REMEMBER: **Plagiarism** is a form of **Academic Dishonesty** which carries harsh consequences. DO YOUR OWN WORK!


PROCEDURE:

Using the data tables on the accompanying handout, RECORD OBSERVATIONS and/or DATA for each procedure step.

Part A: Properties of Hydrates - Qualitative

  1. Place about 0.1 g of CoCl2 · 6H2O in a clean, dry test tube.
  2. Describe the hydrate crystals and record your observations in your Google Document.
  3. Heat the test tube gently over a Bunsen burner flame and record your observations.
  4. After the sample has cooled, add a few drops of distilled water. Record your observations.
  5. Repeat the above steps for: NiCl2 · 6H2O and CuCl2 · 6H2O.

Part B: Formula of a Hydrate - Quantitative


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  1. Turn the hot plate to HIGH and allow to heat up.
  2. Clean a porcelain evaporating dish with soap and water. Dry the dish by placing it on the hot plate for a few minutes.
  3. Carefully remove the evaporating dish with forceps and let cool. Handle the dish with forceps for the remainder of the experiment.
  4. Measure the mass of the cool evaporating dish to the nearest 0.01 g. Record.
  5. Add about 4 g of CuSO4 crystals to the evaporating dish and measure the mass to the nearest 0.01 g. Record.
  6. Place the evaporating dish on the hot plate and heat for about 10 minutes.
  7. Remove the evaporating dish from the hot plate, let it cool in a desiccator (if available) and measure the mass.
  8. Reheat the dish and contents for a few minutes, cool, and measure the mass again. If this mass value does not agree within 0.02 g with the last mass reading, you must reheat and re-measure the mass until the last two measurements are within the designated 0.02 g range.


Disposal and Clean Up

  1. Discard any solids in the trash.
  2. Clean all glassware and return to the proper place.
  3. Clean all work surfaces and personal protective equipment as directed by your instructor.
  4. Wash your hands thoroughly before leaving the laboratory.


ANALYSIS

Use the tables in the approved handout to organize your observations and data described in the procedure.

In the Qualitative part of this activity, you will observe several hydrates both before and after heating. Be sure to record all your observations.

In the Quantitative part of this activity, you will measure the mass of the Copper (II) Sulfate hydrate both before and after heating. This will allow you to investigate the calculate the formula of the hydrate.
CLEARLY SHOW YOUR WORK FOR EACH OF THE 7 PARTS OF THE CALCULATIONS TABLE.



FOLLOW UP QUESTIONS

Write answers for the questions. Remember to use whole sentences. Pay close attention to grammar, spelling, and punctuation.