From Hydrate to Anhydrate: Percent Composition

Transcription

1 Artist as Chemist Section 5 From Hydrate to Anhydrate: Percent Composition What Do You See? Learning Outcomes In this section you will Identify an unknown hydrate. Distinguish between a hydrated and an anhydrous compound. Examine and describe the effects of heat on clay. What Do You Think? Ceramics are materials made from clay and have been used by humans for both practical and artistic purposes dating back almost 13,000 years. However, ceramics are far from being antiquated. In fact, they are used in some of our most high-tech materials today. List as many ceramic materials and products as you can. What are some properties of ceramic materials that make them so useful? What are some properties of ceramic materials that can limit their usefulness? Record your ideas about these questions in your log. Be prepared to discuss your responses with your group and the class. Investigate Part A: Heating a Hydrate If a substance contains water as part of its crystalline structure, it is called a hydrate. The solid that is left when the water is removed from a hydrate is called an anhydrate. In this part of the investigation, you will remove the water from a mystery hydrate. By measuring the amount of water removed, you will be able to identify the hydrate. 230

2 Section 5 From Hydrate to Anhydrate: Percent Composition 1. You will use heat to drive off the water from a hydrate. Dehydrating the hydrated compound requires intense heating for a period of time. You will use a clean, dry crucible and lid to do this. Set up your equipment as shown in the diagram. 2. You will need to determine the mass of water in your hydrate. a) What information will you need to collect in order to calculate this? Design a procedure you will follow to obtain the information. Use the following steps to guide you. b) Construct a data table for recording this information in your log. Make sure that you identify on your data table which unknown you are testing. Have your procedure and data table approved by your teacher before starting. 3. Light the burner. Heat the crucible with its cover in the hottest part of the flame for 3 minutes in order to remove any moisture that might be present in the empty crucible or lid. Remove the crucible carefully using tongs. Place it on a heatresistant surface on your lab table to cool. Turn off the burner. When the crucible and lid have cooled, determine the mass to the nearest 0.01 g. 4. Obtain your unknown hydrate. You will not need more than 2.00 g of the hydrate. Observe the hydrated compound with a hand lens. a) Measure the mass of the unknown hydrate, crucible/lid to the nearest 0.01 g. Record the letter of your unknown hydrate on your data table. b) Describe the crystalline structure of the hydrate in your Active Chemistry log. 5. Place the crucible on the clay triangle. The lid should tilt slightly, which will allow water vapor to escape as it forms. Light your Bunsen burner and adjust flame. 6. Begin heating gently, gradually increasing the heat until there is no more popping or spattering. Remove the cover using tongs and examine the material in the crucible. If the edges of the solid are turning brown, reduce the heat momentarily and then begin heating again at a slower rate. 7. Use tongs to remove the crucible, lid, and contents. Allow them to cool before making any measurements. Some anhydrous compounds will readily absorb moisture from the air so it is important that you quickly determine the mass as soon as it has cooled down. If a desiccator is available, place your crucible inside the desiccator to let it cool. Safety goggles and a lab apron must be worn at all times in a chemistry lab. Follow all safety rules for working with an open flame. Wear protective gloves when handling hot objects. Handle objects with care once they have been heated. Objects look the same whether they are hot or cool. 231

3 Artist as Chemist Wash your hands and arms thoroughly after the investigation. 8. Using good technique and making accurate measurements will be critical in obtaining accurate calculations. Make sure you record mass to the nearest 0.01 g. a) How can you be sure that all of the water has been removed from the hydrated compound? b) Adjust the procedure as needed to ensure that you have removed all of the water. 9. Observe the dehydrated compound with a hand lens. a) Describe the crystalline structure. Note any changes in your observations before and after heating. b) Explain what might be responsible for any changes that you noted. Record these observations and thoughts in your log. 10. Dispose of the materials as directed by your teacher. Clean up your workstation. Part B: Using Calculations to Find the Formula of a Hydrate In this part of the investigation, you will determine the formula of the unknown hydrate. 1. Using the original mass of the hydrate and the final mass of the anhydrate, calculate the mass of the water removed. 2. To determine the identity of the unknown hydrate, you will have to know the amount of water and the amount of hydrate in your original sample. You already know the amount as measured in grams. What you will need to calculate is the amount in moles the wonderful chemistry way of counting. The mole is a specific unit of measurement that chemists use. A mole is simply a quantity of something. It happens to be a very large quantity. Just like a pair means two items and a dozen means 12 items, a mole means items! This odd unit is used because it makes calculations easier. Typically, it will refer to a number of particles atoms, molecules, and ions. One mole of any element is also equal to its atomic mass in grams. From the periodic table, you find that one mole of hydrogen atoms has a mass of g. A mole of carbon atoms has a mass of g, and a mole of oxygen atoms has a mass of g. You can determine the molar mass of a molecular or ionic compound by simply adding up the atomic masses (in grams) of each element present in the molecules or ions. Thus, the molar mass of water is 18.0 g. One mole of H 2 O contains two moles of hydrogen atoms (2.0 g, rounded off) and one mole of oxygen atoms (16.0 g) for a total molar mass of 18.0 g /mol. a) Calculate the amount of water removed from your sample in moles. If you had removed 18 g of water, that would have been 1 mol. If you had removed 0.18 g, that would have been 0.01 mol. _ g of water 1 mol H 2O _ mol H 18.0 g 2 O 3. You do not know which anhydrate you have out of the following four possibilities shown in the table. Therefore, you will need to calculate the number of moles of each anhydrate listed. a) Prepare a similar table in your log. 232

4 Section 5 From Hydrate to Anhydrate: Percent Composition b) Calculate and record the molar mass for each of the possible anhydrates. Suppose your unknown has the formula MgSO 4. The formula tells you several things. The formula indicates that for every one unit of this compound, there is 1 magnesium atom, 1 sulfur atom, and 4 oxygen atoms. You can look up these atomic masses on the periodic table. The first entry is already listed for you, so that you can check your procedure. You can now proceed to calculate the molar masses for the other anhydrates. Anhydrate Molar mass (g/mol) MgSO Na 2 CO 3 CaSO 4 CuSO 4 Number of moles c) Calculate and record the amount of anhydrate in moles. The number of moles must be based on the amount of anhydrate you had divided by the molar mass of each anhydrate. For example, if the mass of the anhydrate was 1.2 g and the anhydrate was MgSO 4, then the amount in moles is 0.01 mol since 1.2 g/(120 g/mol) = 0.01 mol. g of MgSO 4 1 mol MgSO 4 mol MgSO g 4 d) Calculate the amount of anhydrate in moles for each of the anhydrates listed. Use the number of grams of the anhydrate that you measured in the investigation and the molar mass that you just determined. Place these values in the third column of the table. The number of grams will correspond to a different number of moles for each of the four anhydrates listed. 4. You will now make a prediction of your anhydrate based on one more bit of information. Hydrates are formed with specific ratios of the anhydrate and water. MgSO 4 7H 2 O 1 mol of magnesium sulfate combines with 7 mol of water Na 2 CO 3 H 2 O 1 mol of sodium carbonate combines with 1 mol of water CaSO 4 2H 2 O 1 mol of calcium sulfate combines with 2 mol of water CuSO 4 5H 2 O 1 mol of copper sulfate combines with 5 mol of water Using your calculated quantities for moles of water and moles of the possible anhydrates, determine which of the hydrates you began with by computing which of these anhydrates have the correct ratio of anhydrate to water. The ratio is a ratio of moles the chemist s way of counting. a) Record your answer in your log. 5. Compare your results with other members of your class with the same unknown. a) Are there differences? What reasons could account for those differences? 233

5 Artist as Chemist Safety goggles and a lab apron must be worn at all times in a chemistry lab. Wash your hands and arms thoroughly after the investigation. Part C: Clay 1. Obtain a sample of artist s clay from your teacher. Observe the hydrated compound with a hand lens. a) Describe any solid structures you see. 2. Measure the mass of the hydrated clay. a) Record the mass in your Active Chemistry log. 3. Create an object from the clay. You may wish to create something that you will be able to use in your museum display or a replica of another object made of clay. Or you may want to just be creative and make something unique with your clay. 4. Let your object air-dry overnight, or, if available, set it in a low-heat drying oven overnight. You now have dehydrated clay. (We limit the use of the word anhydrate for pure compounds. This clay is composed of many different compounds.) Observe your dried object with a hand lens. Note any changes compared to dehydrated clay, the hydrated clay, and the dehydrated clay object. a) How might you explain any differences that you noted? 5. Measure the mass of the dehydrated clay object. a) Record the mass in your Active Chemistry log. 6. Calculate the mass percent of water in your sample: mass of water removed 100% _% water starting mass of hydrate a) Record the mass percent of water in your log. b) If you had fired your clay in a kiln, how might that affect the calculated percentage of water? 7. Dispose of the materials as directed by your teacher. Clean up your workstation. 234

6 Section 5 From Hydrate to Anhydrate: Percent Composition HYDRATES AND ANHYDRATES Chem Talk Defining Hydrate and Anhydrate Many compounds form as a result of reactions that occur in water solutions. These compounds appear to be dry, but when they are heated, water is released. The water molecules are a part of the crystalline structure and are weakly bonded to the ions or molecules that make up the compound. When a potter fires his clay in a kiln, water is removed from the clay, resulting in a change in the nature of the clay. If a substance contains water as part of its crystal structure, it is called a hydrate. The solid that is left when the water is removed from a hydrate is called an anhydrate. The Chemistry Way of Counting Moles Since atoms are so incredibly small and samples of matter typically contain so many atoms, chemists needed to establish a method for counting atoms. The method is the same one employed by food manufacturers everywhere. No one at the factory would want to have to count the candies that go in a 454-g bag each time. Instead, if they know that 10 candies have a mass of 8.9 g, they can estimate that there will be about 510 candies in a 454-g bag. They can simply use the mass to determine the number of items in the bag. Chem Words hydrate: a compound that has water bonded to it. anhydrate: a compound that does not have any water bonded to it. mole: the number equal to the number of carbon atoms in exactly 12 g of pure 12 C. Chemists routinely use a unit of measurement called the mole. A mole is simply a quantity of something, just like an octet is a specific quantity of something. An octet means eight and a mole happens to mean dozen = 12 1 mole = dozen pots 12 pots 1 mole pots pots 1 dozen atoms 12 atoms 1 mole atoms atoms 1 dozen molecules 12 molecules 1 mole molecules molecules ,200,000,000,000,000,000,

7 Artist as Chemist Chem Words Avogadro s number: the number equal to the number of carbon atoms in exactly 12 g of pure 12 C, units. molar mass: the mass of one mole of a pure substance. Furthermore, if a student has two dozen brushes, you know she has 24 brushes. You carry out the mathematics of this in your head with no trouble. The math looks like this: 12 brushes 2 dozen brushes 1 dozen brushes 24 brushes If you have 2 mol of atoms: 2 mol atoms atoms 2( mol atoms 23 atoms) x atoms By definition, one mole is equal to the number of carbon atoms in exactly 12 g of pure carbon-12 (the isotope carbon-12, or 12 C). Various techniques have been used to determine this number as This number is called Avogadro s number to honor Amedeo Avogadro's contributions to chemistry. It is also true that 12 g of 12 C contains atoms. One mole of any substance will always contain chemical units of that substance. The masses of one mole of two different substances will not be the same because the atoms making up those substances have different atomic masses. The mass of one mole is equal to an element s atomic Amedeo Avogadro mass in grams. Using the periodic table, this means that: 1 mol of He has a mass of g and contains atoms 1 mol of Al has a mass of g and contains atoms 1 mol of Cl 2 has a mass of g and contains atoms For compounds, the atomic mass of each element is added to get the formula weight. For example, 1 mol of H 2 O has a mass of g and contains atoms 2 [H (1.01 g)] 1 [O (16.00 g)] g 1 mol of Zn(SO 4 ) 2H 2 O has a mass of g and contains atoms 1 [Zn(65.4 g)] 1 [S (32.1 g)] + 6 [O (16.0 g)] 4 [H (1.01 g)] g See the pattern? You just add up the masses of all the elements present in a compound to determine the mass of one mole of that compound. This is called the molar mass. This means that anything less than that mass will be only part of a mole. 236

8 Section 5 From Hydrate to Anhydrate: Percent Composition Percent Composition, Empirical Formula, and Molecular Formula Chemists can use percent composition data to calculate a formula for an unknown compound. The percent composition often can be obtained from analysis of the various elements that the substance contains. For example, suppose that a white solid is known to contain 39.3 percent sodium and 60.7 percent chlorine. The procedure for using this data is as follows: Divide the percentage of each element by the element s atomic mass. This gives a value of = 171. for Na and 1.71 for Cl. Next, divide each number by the smaller number. (In this case, the numbers happen to be the same). The result is 1 and 1, or 1:1. The two elements combine in the ratio of 1:1, or NaCl. This gives the empirical formula, which is the formula for the smallest possible ratio of the elements. In this simple example, you know that sodium and chlorine combine in a 1:1 ratio, and sodium chloride is a well-known compound. Here is a more realistic problem: A sample of a white solid is analyzed and found to have 40.0 percent carbon and 6.7 percent hydrogen. Because the amount of carbon and hydrogen do not add up to 100 percent, there must be other elements missing from the percent composition. When the percent composition does not add up to 100 percent, scientists assume that oxygen accounts for the missing element. In this case, you could assume that oxygen would account for the missing 53.3 percent. Dividing each percent by the atomic mass of the element, yields the numbers C= 333., H= 67., and O= Dividing each number by the smallest, 3.33, leaves the empirical formula, CH 2 O. To determine the actual molecular formula, more information is needed. Other analysis shows that the total molecular weight of this compound is 180 g/mol. To determine the molecular formula of the unknown, the molecular mass (180) is divided by the empirical mass (30), giving the number 6. This tells you that 6 units of CH 2 O are needed to obtain the true molecular mass and the true molecular formula, C 6 H 12 O 6. This is the formula for a sugar called glucose. Chem Words percent composition: the mass of each element in a compound expressed as a percent. empirical formula: the formula for the smallest possible ratio of the elements in the compound. molecular formula: a description of the number and types of atoms in a molecule, but not how they are connected. Hydrogen atoms Number of Moles of Each Atom in CaSO 4 2H 2 O Oxygen atoms Sulfur atom Calcium atom CaSO 4 2H 2 O 237

9 Artist as Chemist Checking Up 1. What is a mole? 2. What is molar mass? 3. What is the difference between a compound that is hydrated or anhydrous? 4. How many grams are there in 1.5 mol of water? Sometimes a chemist will know the molecular formula and need to calculate the percent composition. For example, you might have a compound with the formula ZnSO 4. To calculate the percent composition, simply add up the total mass of each element, and then the total mass of the compound. 1[Zn (65.4 g)] 1[S (32.1 g)] 4 [ O (16.0 g)] g Divide the total atomic mass of each element by the total mass of the compound, and multiply by 100 to make it a percent. For zinc, % 40. 5% ; For sulfur, % 19. 9% ; For oxygen, % 39. 6%. A good check on the math is to add up the percentages, which should equal 100% % 19. 9% 39. 6% 100% Putting Hydrates and Anhydrates to Work Gypsum is a natural mineral whose chemical name is calcium sulfate dihydrate. The formula is represented as CaSO 4 2H 2 O. This tells you that each molecule of calcium sulfate has two water molecules attached to it. If you have ever had a broken bone, the doctor may have made a cast out of gypsum to fit over the broken area. Improved materials have replaced gypsum in many places. Gypsum is also called plaster of Paris because it first came from gypsum quarries located near Paris. To make plaster of Paris you start with powdered gypsum by heating the hydrated calcium sulfate to about 160ºC to drive off some of the water to form the hemi-hydrate calcium sulfate (CaSO H 2 O). Hemi means 1 2. Now when you mix it with water it makes a paste that you can apply to the area that has the broken bone. A hard cast is formed when the hemi-hydrate (CaSO H 2 O) reacts with the water to return to the original dihydrate (CaSO 4 2H 2 O). 2CaSO H 2 O 3H 2 O 2CaSO 4 2H 2 O This reaction takes about 30 minutes to set. Plaster has many uses in the world of art as well. Plaster is used to produce intricate details in interior architecture. Plaster can be poured into casts to create sculptures or other pieces of art. Plaster is often used as an intermediate stage for large bronze sculptures. Plaster can also have other substances like cement, sand, and even wood fibers added to it to give it more strength. 238

10 Section 5 From Hydrate to Anhydrate: Percent Composition What Do You Think Now? At the beginning of the section you were asked the following: List as many ceramic materials and products as you can. What are some properties of ceramic materials that make them so useful? What are some properties of ceramic materials that can limit their usefulness? What is a potter doing when she is firing her pottery? What effect does this have on the pottery? What does it mean? Chemistry explains a macroscopic phenomenon (what you observe) with a description of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Complete the chart below in your log. MACRO NANO SYMBOLIC How did the mass measurements of the unknown compound that you investigated convince you that the compound was hydrated? Chem Essential Questions Explain how an anhydrous compound becomes hydrated at the nano level. Use formulas to show the difference between a hydrated and a dehydrated compound. How do you know? Draw a model of the copper sulfate pentahydrate (CuSO 4 5H 2 O) and copper sulfate anhydride using diamonds to represent the anhydride and circles to represent H 2 O molecules. Why do you believe? When measuring anhydrous compounds on the balance, why must you always remember to immediately replace the lid on the reagent container? Why should you care? Would it be possible to incorporate an anhydrate/hydrate chemical system into a work of art? That is, can you imagine a scenario where a color change takes place in a piece of art? Design a simple system based on this concept to be used in some art form. 239

11 Artist as Chemist Reflecting on the Section and the Challenge So far, you have examined several different artistic media, and some of the chemistry behind each one. In this section you investigated hydrated compounds and saw the changes they undergo upon dehydration and re-hydration. You learned that removing water from a hydrated compound affects the crystalline structure of the compound and in turn its properties. You showed these changes by writing formulas and determining the molar masses of the hydrated and anhydrous compounds. Since your challenge is to create a museum display (including a work of art) and to explain the chemistry related to creating it, consider how you might use the knowledge gained from this section in your display. 1. How is knowing the percentage by mass of water different from knowing the mole ratio of water to the hydrate? 2. Why do chemists use the unit moles? Chem to Go 3. What is the gram-molecular mass of NaN 3 (s)? 4. What is the total number of moles of NaN 3 (s) in a 52-g sample of the compound? 5. A sample of a compound contains 65.4 g of zinc, 12.0 g of carbon, and 48.0 g of oxygen. What is the mole ratio (simplest formula) of zinc to carbon to oxygen in this compound? 6. A sample of a substance containing only magnesium and chlorine was tested in the laboratory and was found to be composed of 74.5% chlorine by mass. If the total mass of the sample was g, what was the mass of the magnesium? 7. Kaolinite is a type of clay commonly used for making china. Its formula is Al 2 Si 2 O 5 (OH) 4. Suppose you were given approximately 250 g of kaolinite for creating an object. How many moles of kaolinite did you have? 8. You may have noticed that most electronic equipment comes packed with small packets of desiccants (drying agents that pick up water molecules). Speculate about what you think these desiccants might be made of and why they are placed in packaged electronic equipment. 9. In Part A, if the empty crucible was not heated long enough to completely dry it prior to weighing, what would be the effect on the calculated percent water in the unknown (larger or smaller)? Explain. 10. Explain how determining the percent composition of water might be useful to a potter. 11. Calculate the mass of water removed in forming the anhydrate from 3.22 g of Na 2 SO 4 (10H 2 O). 12. Calculate the number of water molecules removed in forming the anhydrate (from Question 11) from 3.22 g of Na 2 SO 4 (10H 2 O). 240

12 Section 5 From Hydrate to Anhydrate: Percent Composition 13. Gypsum is composed of 23.3 percent calcium, 18.7 percent sulfur, 37.2 percent oxygen, and 20.9 percent water. Calculate the empirical formula. The formula weight of gypsum is 172. Calculate the true formula for gypsum. Show your work. 14. Preparing for the Chapter Challenge Your challenge for this chapter involves the creation of a work of art. In Part C of this investigation you made an object out of clay. How could you use this object in your final work of art? Consider the object and come up with some ideas for how you might add another element to the object to make it more useful or aesthetically pleasing. Take one of your ideas for adding on to the clay object and then do it. Inquiring Further 1. History of ceramics Research the history of ceramics. What can archaeologists discern about a culture or a society from its pottery? How does the history of ceramics also mirror the advancements in technology for a culture? What are some of the newest uses for ceramics today? What is it about the properties of clay that make it so useful? 2. Desiccants Desiccants are used for a variety of purposes because of their ability to absorb moisture. Conduct some research, both in and out of the lab, to determine if there is a relationship between the percent of water in a hydrate and its water-absorbing capacity. Based on your findings, make some recommendations about which anhydrous salts make the best desiccants. 241