What s in a Mole? Molar Mass

Transcription

1 LESSON 10 What s in a Mole? Molar Mass OVERVIEW Key Ideas Lesson Type Lab: Groups of 4 Chemists compare moles of substances rather than masses because moles are a way of counting atoms. When considering toxicity, or other properties of a substance, it is usually the number of particles that matters. The molar mass of a compound depends on the molar masses of the elements that make up the compound. The mass of a substance can be converted to moles using molar mass values from the periodic table. As a result of this lesson, students will be able to calculate the molar mass of compounds describe the approximate magnitude of a mole of a substance complete simple conversions between mass and moles Focus on Understanding The mole is a difficult unit for students to grasp. This lesson assists by having students create tangible molar samples. What Takes Place In this lab, students measure out a mole of several different substances, from water to aluminum, to gain a conceptual grasp of the quantity of a mole and to compare moles of different substances. In Part 1 of the worksheet, students figure out the molar mass of some common substances. In Part 2, they work in groups to create molar samples of several substances. Materials student worksheet nickels (25) aluminum cans (14) 2 g iron nails (40), or about 56 g of nails table salt, 100 g water, 250 ml balance (1 per station) beaker or cup for weighing water weighing boat or beaker to measure salt 50 ml graduated cylinder for water 72 Living By Chemistry Teacher Guide Unit 4 Toxins

2 Setup Before class, set up five weighing stations, one substance at each station: nickels, aluminum cans, water, table salt, and iron nails. You might want to set up more stations to avoid congestion. In addition, you might want to have 1-mole samples of each substance set aside for comparison. The amounts listed for the substances in the materials list are purposefully greater than 1 mole for each substance. This is to prevent giving away the amount needed for 1 mole. You can adjust according to your needs. Cleanup Recycle materials, or save for reuse. Section II Lesson 10 What s in a Mole? 73

3 LESSON 10 What s in a Mole? Molar Mass GUIDE Engage (5 minutes) Key Question: How can you convert between mass and moles? ChemCatalyst Consider 12 nickels, 2 empty aluminum cans, and a balloon full of carbon dioxide gas. a. Which has the greatest mass? b. Which has the greatest number of atoms? c. Which has the greatest number of moles of atoms? Explain the reasoning behind your answers. Sample Answers: Students can consult the periodic table to determine molar masses. Carbon dioxide has a molar mass of g/mol; aluminum has a molar mass of g/mol; nickel has a molar mass of g/mol. However, if you don t know the mass of each sample, the masses cannot be converted to moles or atoms. Discuss the ChemCatalyst Stimulate a discussion on the relationship among mass, moles, and numbers of atoms. Sample Questions What did you base your predictions on? How could you find out which set of objects has the greatest mass? the greatest number of moles of atoms? How can you convert between mass and moles? What makes predictions about carbon dioxide different from predictions about nickel or aluminum? Explore (15 minutes) Introduce the Lab Pass out worksheets. Briefly explain the procedure. Students can do Part 1 individually. Students work in groups of four on Part Living By Chemistry Teacher Guide Unit 4 Toxins

4 LESSON 10 What s in a Mole? Molar Mass Name Date Period LAB Part 1: Molar Mass 1. Use a periodic table to complete the second column in the table. You will complete the last column in Part 2. Chemical formula Molar mass g/mol Mole of what? Equivalent to: Cu(s) g Cu atoms 50 ft of 20-gauge copper wire O 2 (g) g O 2 molecules 22.4 L oxygen gas at STP Ni(s) Al(s) H 2 O(l) g Ni atoms 12 nickel coins g Al atoms 2 aluminum cans H 2 O molecules He(g) He atoms 22.4 L helium gas at STP NaCl(s) g NaCl units 1_ cup salt 4 Hg(l) Hg atoms 14.7 ml mercury Fe(s) g 4.00 g g g g Fe atoms C 12 H 22 O 11 (s) sugar molecules 0.75 lb sugar *Answer will change according to size of nails. 2. How many copper atoms are in g of copper? 602 sextillion copper atoms 18 ml water 28 nails* 3. Copper has a larger molar mass than aluminum. Explain what this means. Individual copper atoms are heavier than individual aluminum atoms. If you have the same number of both kinds of atoms, the copper sample will have more mass. 4. Which contains more moles, 1.0 g Al(s) or 1.0 g Hg(l)? Explain your thinking. One gram of aluminum has more moles than one gram of mercury. Aluminum atoms are lighter, so it takes more aluminum atoms to make up a gram. 5. Explain how you determined the molar mass of sugar. Show your work. Determine the molar mass of each type of atom in the compound. Then multiply by the number of atoms of each type. C: g/mol; H: g/mol; O: g/mol. 12(12.01) + 22(1.008) + 11(16.00) = g/mol Living By Chemistry Teaching and Classroom Masters: Units 4 6 Unit 4 Toxins Key Curriculum Press Lesson 10 Worksheet Section II Lesson 10 What s in a Mole? 75

5 6. Explain why the table says NaCl units. Sodium chloride is an ionic substance. It is not found as molecules or individual atoms. 7. What do you think the volume of a mole of carbon dioxide gas would be? Explain your reasoning. As we learned in Unit 3: Weather, 1 mole of any gas at STP occupies the same volume, 22.4 L. Part 2: Mole Challenge Procedure 1. There are weighing stations around the room. With your group, visit the stations and create 1-mole samples of the items found at each station. These samples must be close to a mole, but they do not have to be exact. 2. Bring your mole samples back to your desk. Be ready to show them to your teacher and to explain how you determined the amount of each substance. 3. Enter the amounts in the last column of the table in Part 1. Questions (Show your work.) 1. How many moles of aluminum do you need to make a six-pack of cans? Two cans are equivalent to about 1 mole of aluminum, so six cans require about 3 moles of aluminum. 2. How many iron nails do you need in order to have g of iron atoms? g is four times g. If 28 nails weigh g, then you need four times that, or 112 nails. 3. What volume of water do you need in order to have 25 moles of H 2 O molecules? If 1 mole of water is equivalent to approximately 18 ml, then you need 25 times 18 ml, or 450 ml of water, to have 25 moles. 4. Making Sense Suppose you have 1 g of sugar and 1 g of water. Which one has more molecules? Explain how you arrived at your answer. A gram of water will have more molecules. A water molecule has much less mass than a sugar molecule, so it takes more molecules to make a 1 g sample. 5. If You Finish Early The average teenager drinks 868 cans of soda per year. Determine how many aluminum cans per year this represents for your school by estimating the number of students. How many moles of aluminum atoms does this represent? Answers will vary depending on the school population. For a school of 1,000 students, the number would be 868,000 cans per year. It takes about two aluminum cans to make 1 mole of aluminum atoms. Therefore, the number of aluminum cans should be divided by 2 to get the number of moles. In this case, the answer is 434,000 moles of aluminum atoms. 30 Unit 4 Toxins Living By Chemistry Teaching and Classroom Masters: Units 4 6 Lesson 10 Worksheet 2010 Key Curriculum Press 76 Living By Chemistry Teacher Guide Unit 4 Toxins

6 Explain and Elaborate (15 minutes) Discuss the Molar Mass of Compounds Sample Questions What do you need to know in order to figure out the mass of a mole of any compound on the planet? How could you figure out the molar mass of carbon dioxide, CO 2? Why is the molar mass listed for oxygen in the periodic table different from the molar mass of oxygen gas? Key Point You can figure out the mass of 1 mole of any element or compound using a periodic table. For compounds, you must sum the molar masses of all the atoms in a molecule or a formula unit to find the molar mass of the compound. For example, each molecule of water contains two atoms of hydrogen and one atom of oxygen. The molar masses of hydrogen and oxygen are g/mol and g/mol, respectively. So the molar mass of water, rounded to the correct significant digits, is g/mol. Compare a Mole s Worth of Different Substances You might want to display a mole of each substance used for the lab. Sample Questions What does 1 mole of water look like? What is its volume? (1 mole is 18 g, which has a volume of 18 ml when liquid about the amount of an ice cube.) What volume, in milliliters would 10 mol of water occupy? (180 ml) How many molecules of water are in your 1 mol sample? How many atoms of aluminum are in a 1 mol sample? Which substance occupies the greatest amount of space per mole of substance? (any of the gases) the least? (the densest substance) What is the volume of 1 mole of any gas at STP? (22.4 L) Which has more mass, 1 mole of oxygen molecules, O 2, or 1 mole of aluminum atoms, Al? (oxygen molecules) Key Points A mole of atoms or molecules of a solid or a liquid is an amount you usually can hold in your hand. The mass per mole of the different elements in the periodic table ranges from g/mol for hydrogen atoms to g/mol for uranium atoms. Two aluminum cans contain about one mole of aluminum atoms. It is a convenient amount of substance to work with. A mole of any gas, if it is at standard temperature and pressure, always has a volume of 22.4 L. Recall that this is Avogadro s law. So a mole of oxygen molecules occupies the same volume as a mole of helium atoms or a mole of chlorine gas molecules. A mole of a gas occupies a fairly large space equivalent to ten 2 L soda bottles because there is so much space between gas molecules. This is why it is difficult to compare the volume of a mole of a gas, like oxygen, and the volume of a mole of a solid, like aluminum. Section II Lesson 10 What s in a Mole? 77

7 Wrap-up Key Question: How can you convert between mass and moles? The molar mass of a compound is the sum of the atomic masses of the atoms in the compound. The molar mass of a compound allows you to convert between moles of the compound and grams of the compound. One mole of a solid or a liquid is an amount that you usually can hold in your hand. One mole of a gas occupies a volume of 22.4 L. This is larger than the volume of 1 mol of a liquid or a solid because of the space between gas molecules. Evaluate (5 minutes) Check-in You have 1 mol of oxygen molecules, O 2, and 1 mol of carbon dioxide molecules, CO 2. Which has more mass? Which has a larger volume at room temperature? Answer: CO 2 has a larger molar mass. Its molar mass is (16.00), or g/mol. The molar mass of O 2 is 2(16.00), or g/mol. They have equal volumes at room temperature. Homework Assign the reading and exercises for Toxins Lesson 10 in the student text. 78 Living By Chemistry Teacher Guide Unit 4 Toxins