Water of Hydration Version 6.3

Transcription

1 Water of Hydration Version 6.3 Michael J. Vitarelli Jr. Department of Chemistry and Chemical Biology Rutgers University, 610 Taylor Road, Piscataway, NJ I. INTRODUCTION Hydrates are compounds that have water molecules bound to their lattices in stoichiometric ratios. By heating the compounds the water molecules can be removed. By measuring the difference between the mass of the hydrate and an-hydrate the student can determine the empirical formula of the hydrate. An example of a hydrate is copper(ii) sulfate pentahydrate (CuSO 4 5H 2 O). As a hydrate this compound is blue, while its anhydrous form (CuSO 4 ) is white. After heating, the water molecules are no longer bound to the sample leading to an anhydrous substance: CuSO 4 5H 2 O(s) heat CuSO 4 (s) + 5H 2 O(g) (1) During this lab you will receive two different hydrates with unknown hydration states. You will heat these samples and measure the mass loss of water during dehydration. The difference between the original sample mass, the hydrate, and the sample mass after heating, the an-hydrate will give you the mass loss of water. The experimental percent of water in the sample is calculated by dividing the mass loss of water by the original sample mass: Experimental % H 2 O = mass water loss 100% (2) original sample mass Lastly, we need a procedure to determine the unknown hydration state. Consider the following example: Suppose we have 3 moles of CuSO 4 5H 2 O. That yields 3 moles of anhydrous CuSO 4 and 15 moles of H 2 O. If we take the ratio of the moles of H 2 O to the moles anhydrous CuSO 4 we obtain the hydration state. During this experiment the student is not told the hydration state. However, the mass of the anhydrate and the mass of the water in the sample is obtained; the moles of both can subsequently be determined. Finally, taking the ratio of moles of water in the sample to the ratio of the moles of the anhydrate yields the hydration state. II. PRE-LABORATORY QUESTIONS 1. (2 Points) You are in space and running out of water. You do have a great deal of magnesium carbonate pentahydrate. It is possible to extract the water from this. Determine the percent of water by mass in the hydrate magnesium carbonate pentahydrate (MgCO 3 5H 2 O). 2. (2 Points) As an introduction to his class, Severus Snape teaches the first years at Hogwarts about hydrates. He begins with equations representing various dehydrations. Which of the following equations properly represents the dehydration of MgCO 3 5H 2 O?

2 2 A) MgCO 3 H 2 O(s) heat MgCO 3 (s)+h 2 O(g) B) MgCO 3 5H 2 O(s) heat MgCO 3 (s)+5h 2 O(g) C) MgCO 3 5H 2 O(s) heat MgCO 3 (s)+h 2 O(g) D) MgCO 3 5H 2 O(s) MgCO 3 (s)+5h 2 O(g) 3. (2 Points) Severus Snape needs to know how much water is in a sample for his potions class. Calculate the percent of water in a sample if before dehydration the sample weighed g, and g after dehydration. 4. (2 Points) Ron and Hermione begin with 1.50 g of the hydrate copper(ii)sulfate x-hydrate (CuSO 4 xh 2 O), where x is an integer. Part of their practical exam is to determine this integer x. They are working in pairs, though Hermione is doing most of the work. This should be discouraged! After dehydration they find that they are left with 0.96 g of the an-hydrate CuSO 4. What is the unknown integer x. Round the answer to the nearest integer. 5. (2 Points) Suppose Harry begins with the hydrate KAl(SO 4 ) 2 12H 2 O. After dehydration he finds that he is left with 3.0 g of the an-hydrate KAl(SO 4 ) 2. How many grams did he start with? 6. (2 Points) Not only does Snape need to teach his students about hydration states, but also naming hydrates. What is the correct formula for barium hydroxide octahydrate and magnesium sulfate heptahydrate. A) BaOH 8H 2 O and MgSO 4 6H 2 O B) Ba(OH) 2 8H 2 O and Mg 2 SO 4 6H 2 O C) Ba(OH) 2 8H 2 O and MgSO 4 6H 2 O D) Ba(OH) 2 8H 2 O and MgSO 4 7H 2 O E) BaOH 8H 2 O and MgSO 4 7H 2 O 7. (2 Points) Dumbledore decides to gives a surprise demonstration. He starts with a hydrate of Na 2 CO 3 which has a mass of 4.31 g before heating. After he heats it he finds the mass of the anhydrous compound is found to be 3.22 g. He asks everyone in class to determine the integer x in the hydrate: Na 2 CO 3 xh 2 O; you should do this also. Round your answer to the nearest integer. 8. (2 Points) Starting with 5.00 g barium chloride n hydrate yields 4.26 g of anhydrous barium chloride after heating. Determine the integer n. Round your answer to the nearest integer. 9. (2 Points) Now that Snape and Dumbledore has taught you the finer points of hydration calculations they have a slightly more challenging problem for you. Suppose you dissolve 52.2 g of Na 2 CO 3 xh 2 O in enough water to make 5.00 L of solution. The final concentration of the solution was found to be M. Determine the integer x in the hydrate: Na 2 CO 3 xh 2 O. Round your answer to the nearest integer. 10. (2 Points) All students, whether in Hogwarts or in Rutgers must follow proper safety precautions. Which of the following safety precautions must be followed during the water of hydration lab? A) Goggles must be worn at all times.

3 3 B) Gloves must be worn at all times. C) Lab coats must be worn at all times. D) Always keep burners under the hood. E) All of the answers are correct. 11. (2 points) Hydrates take the form: compound nh 2 O. An unknown hydrate has a molar mass of 190. g/mol. If the anhydrate has a molar mass of 100. g/mol, what is the integer n in the hydrate. 12. (2 points) What are the correct formulas for copper(ii) nitrate hexahydrate and lithium chromate dihydrate? A) CuNO 3 6H 2 O and Li 2 CrO 4 2H 2 O B) Cu(NO 3 ) 2 6H 2 O and LiCrO 4 2H 2 O C) Cu(NO 4 ) 2 6H 2 O and LiCrO 4 2H 2 O D) Cu(NO 3 ) 2 6H 2 O and Li 2 CrO 4 2H 2 O III. PROCEDURE Saf ety P recautions: During this experiment and in every experiment the student must wear goggles. The Bunsen burner must be lit under the hood. The hood will carry the unburned gas from the burner and any possible gases liberated from the sample away from the students. The color of hot porcelain looks similar to the color of cool porcelain; always assume the crucible is hot and handle it with tongs. Wash your hands after leaving this and every lab. 1. Prepping the crucibles and crucible lids: Obtain, clean and dry two crucibles and their lids. As mentioned in the safety precautions, the crucibles will get hot when heated and not change color or appear hot. Using tongs, place the crucibles with their lids on your clay triangles. The lid should be offset slightly to allow water vapor to easily escape the crucible. Then heat the crucibles with their lids for about four minutes. The point of this is to make sure the crucibles and their lids are completely dry. Turn the burner off and wait three minutes. THE CRUCIBLES ARE STILL HOT! Using the tongs carry the crucibles to your bench. THE CRUCIBLES ARE STILL HOT! Wait 10 minutes while the crucibles and lids further cool. While the crucibles and lids are cooling proceed to step Obtain 1.0 grams of each hydrate and place each in a separate crucible, after the crucibles have cooled. CuSO 4 x-h 2 O is the blue sample, while CoCl 2 x-h 2 O is the purple sample. Put the crucible lids on top of the crucibles. Weigh each crucible, its lid and the hydrate, record these values in the date table at the top of page 3. In each sample x is an unknown integer. 3. Place each crucible with its lid on a clay support triangle and heat each for five minutes on low flame. After this slightly increase the flame and continue heating for ten minutes. After this time, turn off the burner and wait three minutes. THE CRUCIBLES ARE STILL HOT! Using tongs carry the crucible to your bench and wait 8 minutes for the crucible to cool. Increasing the temperature slowly is to minimize the probability of spattering of the sample during dehydration. The lid is to prevent the loss of sample in case of some spattering. The cobalt chloride hydrate often does spatter

4 if heated rapidly. Lastly, do not let the crucible get red hot since unwanted sample decomposition may occur. 4. Carry the crucible to the balance with tongs, the crucibles still will be slightly hot. Weigh each crucible lid and now an-hydrous sample. Record these values. 5. Repeat steps three and four. Our goal here is to drive off any remaining water in the sample. If the differences in the masses between the two measurements is greater then 0.01 g, steps three and four must be repeated yet again. Record these values and all values in Table I. 6. Keep a note of the colors of the samples before and after desiccation. 4 TABLE I: Data Table Parameter CuSO 4 x-h 2O CoCl 2 x-h 2O Mass of crucible and lid (g) Mass of crucible, lid and sample (g) Mass of sample (g) Mass of crucible, lid and sample No Entry Here No Entry Here after first heating (g) after second heating (g) Mass of water lost (g) Percent of water in sample by mass

5 5 IV. POST-LAB QUESTIONS 1.) What is the percent of water in CuSO 4 x-h 2 O? 2.) What is the percent of water in CoCl 2 x-h 2 O? 3.) CoCl 2 x-h 2 O is the originally purple sample. Calculate the value for the integer x? 4.) Suppose you start with 1.80 g of MgCO 3 5H 2 O, after complete dehydration how many grams of the sample do you have remaining? 5.) Which pairs best represents the colors for before and after dehydration of CoCl 2 x-h 2 O. (Purple, Sky-Blue) (Purple, Pink) (Purple, Orange) (Purple, White) 6.) Assume a desiccator is not used. Where would you prefer to perform this experiment; on Earth or on Mercury? Explain your answer. 7.) You are trapped on a space ship. You have plenty of oxygen, but no water. Though, you do have many kilograms of MgCO 3 5H 2 O. Design a procedure to extract the water.

6 6 V. CHEMICAL HAZARD AWARENESS FORM This form is to be turned in prior to beginning your experiment. Name: Section: RUID: Experiment: List below any chemicals used in this experiment. Also list their hazards and any handling precautions for these chemicals.

7 7 VI. RESULTS: WATER OF HYDRATION Name: Section: RUID: Instructor: Results: Conclusions and Comments: TABLE II: Results Table Parameter CuSO 4 x-h 2O CoCl 2 x-h 2O Mass of water lost (g) Percent of water in sample by mass Percent of anhydrate in sample by mass Hydration state, the integer x