Karen C. Timberlake William Timberlake Fourth Edition

Transcription

1 Basic Chemistry Karen C. Timberlake William Timberlake Fourth Edition

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3 Chemical Quantities LOOKING AHEAD 1 The Mole 2 Molar Mass 3 Calculations Using Molar Mass 4 Mass Percent Composition and Empirical Formulas 5 Molecular Formulas Max, a six-year-old dog, is listless, drinking large amounts of water, and not eating his food. His owner takes Max to his veterinarian, Sean, for an examination. Sean weighs Max and obtains a blood sample for a blood chemistry profile, which determines the overall health, detects any metabolic disorders, and measures the concentration of electrolytes. The results of the lab tests for Max indicate that the white blood count is elevated, which may indicate an infection or inflammation. His electrolytes, which are also indicators of good health, were all in the normal ranges. The electrolyte chloride 1Cl - 2 was in the normal range of mol/l, and the electrolyte sodium 1Na + 2 has a normal range of mol/l. Potassium 1K + 2, another important electrolyte, has a normal range of mol/l. To treat the possible infection, Sean ordered 375 mg tablets of Clavamox, which is a broad-spectrum antibiotic approved for veterinary use in dogs. Career: Veterinarian Veterinarians care for domesticated pets such as rats, birds, dogs, and cats. Some veterinarians specialize in the treatment of large animals, such as horses and cattle. Veterinarians interact with pet owners as they give advice about feeding, behavior, and breeding. In the assessment of an ill animal, they record the animal s symptoms and medical history including dietary intake, medications, eating habits, weight, and any signs of disease. To diagnose health problems, veterinarians perform laboratory tests on animals including a complete blood count and urinalysis. They also obtain tissue and blood samples, as well as vaccinate against distemper, rabies, and other diseases. Veterinarians prescribe medication if an animal has an infection or illness. If an animal is injured, a veterinarian treats wounds, and sets fractures. A veterinarian also performs surgery such as neutering and spaying, provides dental cleanings, removes tumors, and euthanizes animals. From Chapter 7 of Basic Chemistry, Fourth Edition. Karen C. Timberlake, William Timberlake. Copyright 2014 by Pearson Education, Inc. All rights reserved.

4 In chemistry, we calculate and measure the amounts of substances to use in the lab. Actually, measuring the amount of a substance is something you do every day. When you cook, you measure out the proper amounts of ingredients so that you don t have too much of one and too little of another. At the gas station, you pump a certain amount of fuel into your gas tank. If you paint the walls of a room, you measure the area and purchase the amount of paint that will cover the walls. If you take a pain reliever, you read the label to see how many tablets of aspirin or ibuprofen are needed. You read the nutrition label on food packaging to determine the quantities of carbohydrate, fat, sodium, iron, or zinc. Each substance that we use in the chemistry lab is measured so that it functions properly in an experiment. The chemical formula of a substance tells us the number and kinds of atoms it has, which we then use to determine the mass of the substance to use in an experiment. Similarly, when we know the percentage and mass of the elements in a substance, we can determine its formula. LEARNING GOAL Use Avogadro s number to determine the number of particles in a given number of moles. CORE CHEMISTRY SKILL Converting Particles to Moles 1 The Mole At the grocery store, you buy eggs by the dozen or soda by the case. In an office-supply store, pencils are ordered by the gross and paper by the ream. Common terms such as dozen, case, gross, and ream are used to count the number of items present. For example, when you buy a dozen eggs, you know you will get 12 eggs in the carton. Avogadro s Number In chemistry, particles such as atoms, molecules, and ions are counted by the mole 1abbreviated mol in calculations2, a unit called Avogadro s number that contains * items. Avogadro s number is a very big number because atoms are so small that it takes an extremely large number of atoms to provide a sufficient amount to weigh and use in chemical reactions. Avogadro s number is named for Amedeo Avogadro , an Italian physicist.

5 24 cans = 1 case 144 pencils = 1 gross * = sheets = 1 ream 12 eggs = 1 dozen Collections of items include dozen, gross, and mole. One mole of any element always contains Avogadro s number of atoms. For example, 1 mol of carbon contains * carbon atoms; 1 mol of aluminum contains * aluminum atoms; 1 mol of sulfur contains * sulfur atoms. 1 mol of an element = * atoms of that element Avogadro s number tells us that 1 mol of a compound contains * of the particular type of particles that make up that compound. One mol of a molecular compound contains Avogadro s number of molecules. For example, 1 mol of CO 2 contains * molecules of CO 2. For an ionic compound, 1 mol contains Avogadro s number of formula units, which are the groups of ions represented by its formula. For the ionic formula, NaCl, 1 mol contains * formula units of NaCl 1Na +, Cl - 2. Table 1 gives examples of the number of particles in some 1-mol quantities. One mol of sulfur contains * sulfur atoms. TABLE 1 Number of Particles in 1-Mol Quantities Substance Number and Type of Particles 1 mol of Al * atoms of Al 1 mol of Fe * atoms of Fe 1 mol of water 1H 2 O * molecules of H 2 O 1 mol of vitamin C 1C 6 H 8 O * molecules of vitamin C 1 mol of NaCl * formula units of NaCl We can use Avogadro s number as a conversion factor to convert between the moles of a substance and the number of particles it contains * particles 1 mol and 1 mol * particles For example, we use Avogadro s number to convert 4.00 mol of iron to atoms of iron mol Fe * * 1023 atoms Fe 1 mol Fe = 2.41 * atoms of Fe Avogadro s number as a conversion factor We can also use Avogadro s number to convert 3.01 * molecules of CO 2 to moles of CO * molecules CO 2 * 1 mol CO 2 molecules * molecules CO 2 = 5.00 mol of CO 2 molecules Avogadro s number as a conversion factor In calculations that convert between moles and particles, the number of moles will be small compared to the number of atoms or molecules, which will be large.

6 CONCEPT CHECK 1 Moles and Particles Explain why 0.20 mol of aluminum is a small number, but the number of atoms in 0.20 mol is a large number: 1.2 * atoms of aluminum. ANSWER The term mole, abbreviated mol, is a unit that represents * particles. Because atoms are submicroscopic particles, a large number of atoms are present in 1 mol of aluminum. SAMPLE PROBLEM 1 Calculating the Number of Molecules How many molecules are present in 1.75 mol of carbon dioxide? CO 2 molecules The solid form of carbon dioxide is known as dry ice. Guide to Calculating the Atoms or Molecules of a Substance 1 State the given and needed quantities. 2 Write a plan to convert moles to atoms or molecules. 3 Use Avogadro s number to write conversion factors. 4 Set up the problem to calculate the number of particles. SOLUTION Step 1 State the given and needed quantities. Analyze the Problem Step 2 Step 3 Step 4 Given Need 1.75 mol of CO 2 molecules of CO 2 Write a plan to convert moles to atoms or molecules. moles of CO 2 Avogadro>s number molecules of CO 2 Use Avogadro s number to write conversion factors. 1 mol of CO 2 = * molecules of CO * molecules CO 2 1 mol CO 2 and 1 mol CO * molecules CO 2 Set up the problem to calculate the number of particles mol CO 2 * * 1023 molecules CO 2 1 mol CO 2 = 1.05 * molecules of CO 2 STUDY CHECK 1 How many moles of water, H 2 O, contain 2.60 * molecules of water?

7 Moles of Elements in a Chemical Formula We have seen that the subscripts in a chemical formula indicate the number of atoms of each type of element in a compound. For example, one molecule of NH 3 consists of 1 N atom and 3 H atoms. If we have * molecules 11 mol2 of NH 3, it would contain * atoms 11 mol2 of N and 3 * * atoms 13 mol2 of H. Thus, each subscript in a formula also refers to the moles of each kind of atom in 1 mol of a compound. For example, the subscripts in the NH 3 formula specify that 1 mol of NH 3 molecules contains 1 mol of N atoms and 3 mol of H atoms. The chemical formula subscripts specify the NH 3 Atoms in 1 molecule 1 atom of N 3 atoms of H Moles of each element in 1 mol 1 mol of N 3 mol of H Aspirin, C 9 H 8 O 4, is a drug used to reduce pain and inflammation in the body. Using the subscripts in the chemical formula of aspirin shows that there are 9 carbon atoms, 8 hydrogen atoms, and 4 oxygen atoms. The subscripts of the formula of aspirin, C 9 H 8 O 4, also tell us the number of moles of each element in 1 mol of aspirin: 9 mol of C atoms, 8 mol of H atoms, and 4 mol of O atoms. Number of atoms in one molecule Carbon (C) Hydrogen (H) Oxygen (O) Aspirin C 9 H 8 O 4 The chemical formula subscripts specify the C 9 H 8 O 4 Atoms in 1 molecule 9 atoms of C 8 atoms of H 4 atoms of O Moles of each element in 1 mol 9 mol of C 8 mol of H 4 mol of O Using the subscripts from the formula, C 9 H 8 O 4, we can write the conversion factors for each of the elements in one mol of aspirin: 9 mol C 8 mol H 4 mol O 1 mol C 9 H 8 O 4 1 mol C 9 H 8 O 4 1 mol C 9 H 8 O 4 1 mol C 9 H 8 O 4 9 mol C 1 mol C 9 H 8 O 4 8 mol H 1 mol C 9 H 8 O 4 4 mol O