Lab 1 Determination of the Formula of a Hydrate

A P CHEMISTRY Lab 1 Determination of the Formula of a Hydrate Pre-Lab Questions - (Must be completed before lab work begins.) 1. Suppose 2.3754 g of copper (II) sulfate pentahydrate is heated to drive off the water of crystallization. Calculate what weight of anhydrous salt will remain. . 2. In CuSO4 5H2O, it was mentioned that four of the five water molecules held per formula unit of the salt were attached by coordinate covalent bonds to the copper ion. The fifth water molecule is attached to the sulfate ion, but by a different mechanism. Use your textbook or a chemical encyclopedia to determine how a water molecule might be bonded to a sulfate ion.

INTRODUCTION - Many hydrated substances lose their water of crystallization readily if the hydrate is heated to a few hundred degrees. Usually the water of crystallization can be observed as steam escaping from the crystals as they are heated or as a condensate on a cooler portion of the vessel used for the heating. A common experiment in the characterization of new or unknown compounds is to measure the mass of water driven off from a sample of the compound when it is heated. This is true whether the compound is a hydrate or has merely adsorbed water on the surface of its crystals. For a true hydrate, it is often possible to determine the molar ratio in which water is combined in the hydrated compound. Many hydrates, especially those of transition metals, are stable and stoichiometric. That is, they contain a specific, definite amount of water per mole of metal ion present. In this experiment, you will be given a sample of an unknown hydrated metal salt. You will be told what ions are present in the sample, but will not be told how many water molecules are bound in the compound per mole of metal ion. By determining carefully the weight lost on heating a weighed sample of the unknown, you will be able to calculate the percentage of water in the hydrated metal salt. From the percentage, you will be able to determine the number of water molecules bound per metal ion in the unknown hydrate.

SAFETY:  Wear safety glasses at all times while in the laboratory.  Nitric acid (used for cleaning the crucible in this experiment) can burn the skin and can emit toxic fumes if heated. If spilled, wash immediately and inform the instructor.  When the crucible used in the experiment is heated, spattering of the contents may occur if too large a flame is used initially. Heat very slowly at first with a small flame, and do not apply the full heat of the burner until most of the water has been driven from the hydrate.  The unknown metal hydrate may be toxic. Wash hands after handling and report any spills to the instructor. Procedure 1. Record all data and observations directly in your notebook in ink. 2. Obtain a porcelain crucible and a cover that fits the crucible. These items are extremely fragile and expensive. 3. If there is any solid material in the crucible, use a paper towel to wipe out the solid. Wash the crucible with soap and water to clean out any remaining solids. 4. If the crucible cannot be cleaned with soap and water, transfer the crucible to the exhaust hood, and add a few mL of 3 M nitric acid (Caution!). Allow the crucible to stand with the acid for 5-10 minutes. Discard the nitric acid in the hood drain, and rinse the crucible with large amounts of water. If the crucible is still not clean, consult with the instructor. 5. Set up a clay pipe triangle on a metal ring on a ring stand. Wipe the crucible with a paper towel, and then place the crucible on the triangle. Set the cover onto the crucible so that it is slightly ajar. From this point on, handle the crucible only with crucible tongs (when hot) or a paper towel (when cold), to prevent skin

5／7／2018 1 5/7/2018 A P CHEMISTRY Lab 1 Determination of the Formula of a Hydrate oils from adhering to the crucible and changing its mass.

6. Heat the crucible/cover with a small flame until all visible moisture has been removed from the crucible. 7. Increase the size of the flame, and heat the crucible/cover to red heat for 5 minutes. 8. Allow the crucible/cover to cool completely to room temperature. 9. Transfer the crucible and cover (using a paper towel to avoid oils from the fingers) to a watch glass. Weigh the crucible and cover to the nearest mg (0.001 g). 10. Return the crucible/cover to the clay triangle, and reheat in the full heat of the burner for 5 more minutes. 11. Allow the crucible/cover to cool to room temperature and reweigh. If the second weight determination differs from the first determination by more than 5 mg (0.005 g), reheat the crucible and cover for additional 5-minute periods until this level of agreement is reached. 12. Obtain a sample of an unknown metal hydrate (caution) and record its identification number and appearance. Place about 1 g of the hydrate sample into the crucible. 13. Reweigh the crucible, cover, and hydrate to the nearest mg (0.001 g). Place the crucible and its cover onto the clay triangle, arrange the cover so that it is slightly ajar, and begin heating the crucible with a very small flame. 14. Continue heating with a small flame for 5 minutes. Beware of spattering during this time period: if spattering occurs, quickly push the cover over the crucible and remove the flame. 15. After you have heated the crucible gently, most of the water should have been driven from the crystals. Increase the size of the flame somewhat, and heat the crucible and contents for 10 minutes at the higher heat level with the cover slightly ajar. After the heating period, close the cover on the crucible, and allow the assembly to cool to room temperature. 16. When the crucible has cooled completely to room temperature, weigh the crucible, cover, and contents to the nearest mg (0.001 g). 17. Return the crucible to the clay triangle, and reheat the crucible with the cover slightly ajar for an additional 10-minute period. Allow the crucible to cool to room temperature and reweigh. If the second weight determination differs from the first determination by more than 5 mg (0.005 g), reheat for an additional 10- minute period, and reweigh. 18. From the weight determinations, calculate the percentage of water present in the hydrated salt. From the information provided to you about what ions are present in the salt, determine the number of water molecules per formula unit of the salt. 19. Dispose of the sample as directed by the instructor. Clean the crucible and cover, and reheat to constant weight. 20. Use a second 1 g sample of your unknown hydrate to repeat the determination.

5／7／2018 2 5/7/2018 A P CHEMISTRY Lab 1 Determination of the Formula of a Hydrate NAME ______ANSWER SHEET PERIOD _____

Results / Observations Unknown ID number ______Color ______

Ions indicated to be present in the unknown ______Trial 1 Trial 2 Mass empty crucible/cover after first heating ______

Mass empty crucible/cover after second heating ______

Mass crucible/cover/sample before heating ______

Mass crucible/cover/sample after first heating ______

Mass crucible/cover/sample after second heating ______

1. Mass water driven off ______

2. % water in the hydrate ______

3. Mean % water in hydrate ______

4. Formula of the hydrated salt ______

Questions – Answer on the bottom or back of this sheet. 1. Some hydrated salts decompose when heated, rather than just losing the water of crystallization. Use your textbook or a handbook of chemical data to find two examples of such salts. 2. Sugars and starches belong to a class of biological compounds called carbohydrates, indicating that the general formula for such compounds is of the sort (CH2O)n. Use your textbook or a chemical encyclopedia to find out why such compounds are not really hydrates of carbon as the family name suggests.

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