Chemistry 151 Last Updated: Dec Lab 5: Hydrated Compounds

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1 Chemistry 151 Last Updated: Dec Lab 5: Hydrated Compounds Introduction When ionic compounds form, there are sometimes gaps or cavities within the crystal lattice that are large enough to trap water molecules, forming a hydrated compound. For example, Epsom salt is hydrated magnesium sulfate, with seven water molecules per formula unit, MgSO 4 7H 2O. You can often remove the water in a hydrated compound by heating, forming the anhydrous form of the compound. In many cases the anhydrous and hydrated forms of the compound will have distinctly different colors. Chromium (III) chloride, for example, has a dark green color when anhydrous, but is violet as CrCl 3 6H 2O. With many anhydrous compounds, simply absorbing moisture from the air can readily hydrate them. Such compounds are said to be hygroscopic. In fact, there are some hygroscopic compounds that absorb so much moisture that a pool of water will form around the solid. These types of compounds are said to be deliquescent. In this lab you will examine the properties of several different hydrated compounds. You will also experimentally determine the chemical formula of a hydrated compound. Procedure Part I: Sodium Hydroxide 1. Place a pellet of sodium hydroxide on a watch glass and observe it over the course of the lab. Part II: Calcium Chloride 1. Place a small amount of CaCl 2 2H 2O in a medium-sized test tube. 2. Using a Bunsen burner with a cool (yellow) flame, gently heat the tip of the test tube for no more than 30 seconds. Record your observations. 3. Continue heating for another minute or two, until all of the water has been evaporated (including on the sides of the tube) 4. Set the test tube aside and see if any other changes occur. Part III: Cobalt (II) Chloride Repeat Part II using CoCl 2 6H 2O Part IV: Copper (II) Sulfate Repeat Part II using hydrated copper (II) sulfate. Part V: Experimentally Determining the Formula of Hydrated Copper (II) Sulfate 1. Rinse a crucible with 6M nitric acid. 2. Attach an iron ring to a ring stand and set a clay triangle over the ring. Place the crucible in the triangle and heat it to dryness with a Bunsen burner. 3. Once dry, allow the crucible to cool and measure its mass with a lid. 4. Add about 1 gram of hydrated copper (II) sulfate to the crucible and record the combined mass of the crucible, lid and solid. 5. Return the crucible to the clay triangle, set the lid aside, and heat the solid for ten minutes. 6. Remove the heat, cover the crucible with the lid and allow it to cool. 7. When cool enough to handle, record the new mass of crucible, lid and solid. 8. Repeat steps 1-7 for a second determination. Waste Disposal Solid waste should be disposed of in the waste hood. If you have difficulty removing the solid from the test tube, dissolve it with water. Nitric acid can go down the drain with running water.

2 Data Part I: Sodium Hydroxide Part II: Calcium Chloride Part III: Cobalt (II) Chloride Part IV: Hydrated Copper (II) Sulfate

3 Part V: Experimentally Determining the Formula of Hydrated Copper (II) Sulfate Trial 1 Trial 2 1) Mass of crucible and lid, g 2) Mass of crucible, lid and hydrated comp., g 3) Mass of hydrated comp., g 4) Mass of crucible, lid and anhydrous comp., g 5) Mass of anhydrous comp., g 6) Moles of copper sulfate 7) Mass of water, g 8) Moles of water 9) Mole ratio, water to copper sulfate Average (rounded to nearest integer) Formula of hydrated compound: Show your work for the following calculations from Trial #1 a) Mass of water d) Moles of anhydrous compound b) Mass of anhydrous compound e) Ratio of water to compound c) Moles of water

4 Post-lab Questions 1. Which compound(s) studied today appeared to be hygroscopic? Explain. 2. Which compound(s) studied today appeared to be deliquescent? Explain. 3. A desiccator is a special airtight container used to store moisture-sensitive compounds. Usually the bottom of the container is lined with either anhydrous calcium chloride or anhydrous cobalt (II) chloride, which absorbs any residual moisture that s initially present, or any that s introduced after you open or close the desiccator. Once the compound has become fully hydrated, you can either reheat it to return it to its anhydrous state, or simply replace it with a fresh batch. Based on your observations in this lab, which compound calcium chloride or cobalt (II) chloride would be the better choice for this application? Explain. 4. How would the following errors affect the calculated mass of water in Part V? Would it make the value incorrectly high, incorrectly low, or would there be no effect? Explain your answers. a) A student skipped the first two steps and didn t clean the crucible with HNO 3. There were impurities present that were removed as the salt was being heated. b) A student skipped the first two steps and didn t clean the crucible with HNO 3. There were impurities present that weren t removed as the salt was being heated. c) The salt was weighed on a piece of weighing paper then transferred to the crucible, but some of the solid was lost during the process. The crucible, lid and salt weren t reweighed after the transfer to get the combined mass of the three (line 2 of the data sheet). Rather, the recorded mass of the solid (line 3) was simply added to the previously recorded mass of the crucible and lid (line 1).

5 Pre-lab Questions 1. Define the following terms: a) Hydrated salt b) Anhydrous salt c) Hygroscopic d) Deliquescent 2. In Part V, why is the lid removed from the crucible while heating the solid? Why do you place the lid on the crucible when letting it cool? 4. A 3.00 g sample of a hydrated calcium carbonate compound, CaCO 3 xh 2O, was heated. Afterwards, the anhydrous compound weighed 2.21 g. a) Calculate the mass of water in the hydrated compound. b) Calculate the moles of water in the hydrated compound. c) Calculate the moles of calcium carbonate in the hydrated compound. d) Find the ratio of water to calcium carbonate to determine the value of x. e) Write the correct formula of the hydrated compound: 5. Watch the following video on using a Bunsen burner: a) What does adjusting the valve on the bottom do? b) What does adjusting the barrel of the burner do?