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Introductory Chemistry Nivaldo Tro Unit 7 Chemical Composition: The Mole (Chapter 6) 2006, Prentice Hall We Need to Count atoms Airbags are inflated by a chemical reaction: 2 NaN3(s) electrical decomposition 3 N2(g) + 2 Na(s) Each airbag needs the right amount of NaN3 (sodium azide) to fill the bag with nitrogen gas Grouping Items for Counting • The size of the group depends on how many items we use at a time 1 dozen=12 gross = 12x12 1 A Mole is the Counting Unit for Atoms 1 mole = “chemists dozen” 1 pair = 2 items 1 dozen = 12 items 1 gross = 144 items 1 mole = 1 mole eggs = 6.02 x 1023 eggs 1 mole H atoms = 6.02 x 1023 H atoms 1 mole S atoms = 6.02 x 1023 S atoms Average Atomic Mass The original atomic mass scale used 1 H atom as the base. The average atomic mass of 1 H atom = 1.008 amu. The average atomic masses of all other atoms is relative. 1 H atom = 1 proton = 1.7 x 10-24 g = 1 atomic mass unit 1 C atom = 12 x 1.7 x 10-24 g = 2 x 10-23 g = 12 amu How much does 1 mole of atoms weigh? Avogadro’s number was conveniently chosen so that 1 mole of H atoms weighs 1.00 g 1 C atom weighs 12 amu 1 mole of C atoms weighs 12 g 2 Atomic masses have the same relative ratio as molar masses. 1 zinc atom 1 iodine molecule Moles to Grams --> Grams to Moles – Sample A Sample B 1 mol H atoms 6.02 x 1023 atoms Mass = 1.008g ? mol H atoms ? atoms Mass = 0.500g How to calculate the number of moles in 0.500g. 1. Write down the mass of the sample with units and write down the variable with a unit that you are trying to convert to. 2. Write the conversion factor (molar mass of the element) 3. SMILE to get your answer The Mole Review Mass Particles g/mol 1 mol/ 6.02 x 1023 mol/g 6.02 x 1023/ 1mol Moles 3 Moles --> Grams Grams --> Moles • Aluminum (Al), a metal with a high strength-to-weight ratio and a high resistance to corrosion, is often used for structures such as high-quality bicycle frames. Compute the number of moles of atoms in a 10.0g sample of aluminum. Moles --> Grams Grams --> Moles • Aluminum (Al), a metal with a high strength-to-weight ratio and a high resistance to corrosion, is often used for structures such as high-quality bicycle frames. Compute the total number of aluminum atoms in a 10.0g sample of aluminum. Calculations with molar mass Moles Grams • A Si chip used in an integrated circuit of a microcomputer has a mass of 5.68mg. How many silicon atoms (Si) are present in this chip? Set up the question: 5.68 mg Si = ? g Si 4 Mole Review 1. How many moles of Cl are in a 25 g sample of Cl atoms? 2. How many grams of Fe are in 105 moles of Fe? 3. How many moles of Hg are in a 7 ug sample of Hg? 4. How many moles of Cr are in a 1000 kg sample of Cr? How many atoms of Cr is this? Molar Mass of a Compound Molar mass • Molar mass – the mass in grams of one mole of a substance. To calculate the molar mass of a compound, find the number of grams of each element in one mole of the compound. Then add the masses of the elements in the compound. 5 Molar Mass One molecule of SO3 has a mass of 80.1 amu. One mol of SO3 has a mass of 80.1 g. Molar mass Molar mass of sulfur dioxide: SO2 Problem 6 Problem • Calcium carbonate, CaCO3, is the principle mineral found in limestone, marble, chalk, pearls, and the shells of marine animals such as clams. 1. Calculate the molar mass of calcium carbonate. 2. A certain sample of calcium carbonate contains 4.86 mol. What is the mass in grams of this sample? Problem • Isopentyl acetate, C7H14O2, the compound responsible for the scent of bananas, can be produced commercially. Interestingly, bees release about 1 ug of this compound when they sting. This attracts other bees, which then join the attack. How many moles and how many molecules of isopentyl acetate are released in a typical bee sting? Introductory Chemistry Nivaldo Tro Unit 7 Chemical Composition: Percent Composition (Chapter 6) 2006, Prentice Hall 7 Percent Composition • It helps to know the percents of the components in a shirt because they affect how warm it is, whether it will need to be ironed, and how it should be cleaned. You will learn how the percents of the elements in a compound are important in chemistry. Percent Composition from Mass Data • The relative amounts of the elements in a compound are expressed as the percent composition or the percent by mass of each element in the compound. Percent Composition • The percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound, multiplied by 100%. 8 Percent Composition from a Formula Percent Composition by Mass Practice 1. What is the percent by mass of H and the percent by mass of O in water? 2. A compound with a mass of 0.237 g has 0.0948 g C, 0.1264 g O, and 0.0158 g H. What is the % composition of each element? 9 Introductory Chemistry Nivaldo Tro Unit 7 Chemical Composition: Empirical/Molecular Formulas (Chapter 6) 2006, Prentice Hall Empirical vs Molecular Formulas • Ethyne (C2H2) is a gas used in welder’s torches. Styrene (C8H8) is used in making polystyrene. • These two compounds of carbon have the same empirical formula (CH) but different molecular formulas. Empirical Formula • The empirical formula gives the. 10 Calculating Empirical Formulas Determining Empirical Formulas Molecular Formula – The molecular formula of a compound is either the same as its experimentally determined empirical formula, 11 Molecular Formulas Methanal, ethanoic acid, and glucose all have the same empirical formula—CH2O. Finding the Molecular Formula 12 Practice 13