Unit 7A - The Mole. We Need to Count atoms. The Mole and Molar Mass

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1 Unit 7A - The Mole The Mole and Molar Mass We Need to Count atoms Airbags are inflated by a chemical reaction: electrical 2 NaN 3 (s) 3 N 2 (g) + 2 Na(s) decomposition Each airbag needs the right amount of NaN 3 (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

2 A Mole is the Counting Unit for Atoms 1 mole = chemists dozen 1 mole = 6.02 x is 1 mole eggs = 6.02 x eggs 1 mole H atoms = 6.02 x H atoms 1 mole S atoms = 6.02 x S atoms Individual atoms weigh very little 1 carbon atom = 6 protons + 6 neutrons = 12 atomic mass units = 2 x g 1 Mg atom = 12 protons + 12 neutrons = 24 amu = 4 x g How much does 1 mole of atoms weigh? Avogadro s number was conveniently chosen so that 1 C atom weighs 1 mole of C atoms weighs 2

3 The Mole Pyramid Mole Practice How many moles of Hg are in a 7 ug sample of Hg? Grams --> Moles --> Particles 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 atoms in an aluminum frame that has a mass of 9.9 x10 6 g. 3

4 Calculations with molar mass Grams --> Moles --> Particles A Si chip used in an integrated circuit of a microcomputer has a mass of 5.68 mg. How many silicon atoms (Si) are present in this chip? Molar Mass of a Compound Molar mass Molar mass 1 C 4 H + CH x atoms of CH 4 Molar mass = 4

5 Molar mass Molar mass of sulfur dioxide: SO 2 Problem Isopentyl acetate, C 7 H 14 O 2, 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? Unit 7A - The Mole Percent Composition 5

6 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. May also be expressed as: [mass of element sample/mass of compound sample] x 100 Propane, Practice # 2 A 3.52 g sample of chromium reacts with fluorine to produce 7.38 g of the metal fluoride. What is the mass percent composition of fluorine in the fluoride? 6

7 Unit 7A - The Mole Empirical and Molecular Formulas Empirical vs Molecular Formulas Ethyne (C 2 H 2 ) is a gas used in welder s torches. Styrene (C 8 H 8 ) 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 lowest whole-number ratio of the atoms of the elements in a compound. Procedure for calculating the empirical formula given % composition: 1. Convert % of each element to grams, based on a 100 g total 2. Calculate moles of each element by converting from grams to moles using molar mass 3. Determine the mole ratio by dividing through by smallest number of moles 7

8 Determining Empirical Formulas - Practice A laboratory analysis of vanillin, the flavoring agent in vanilla, determined the following mass percent composition: C, 63.15%; H, 5.30%; O, 31.55%. Determine the empirical formula of vanillin. Molecular Formula The molecular formula of a compound is either the same as its experimentally determined empirical formula, or it is a simple whole-number multiple of its empirical formula. Finding the Molecular Formula Aspirin contains 60.0% carbon, 4.48% hydrogen, 35.5% oxygen; molar mass = 180 g/mol. What is the empirical and molecular formula? 8

9 Hydrates A hydrated compound is one in which molecules of water are associated with ions. Common hydrates: 1 Plaster of Paris - CaSO 4 H 2 O Humidity indicator used in packaging of electronics - CoCl 2 XH 2 O 2 Hydrates A g sample of hydrated barium iodide is heated to drive off water; the dry sample has a mass of g. What is the mole ratio between barium iodide and water? What is the formula of the hydrate? What is the name of the hydrate? Unit 7A - The Mole Molarity 9

10 Molarity: Moles in solution Many reactions occur in solution. It is often more convenient to measure the volume of a solution than the mass. The concentration of a solution is the amount of solute in a given volume of solution The most commonly used expression of concentration is molarity (M) M = molarity = Sugar Solutions 1.0 L 1.0 M C 6 H 12 O 6 (glucose) 1.0 L 1.0 M C 12 H 22 O 11 (sucrose) 500 ml 1.0 M C 12 H 22 O 11 (sucrose) 1. Which solution contains the most molecules? Explain. 2. Which solution has the greatest concentration? Explain. 3. Which solution weighs the most? Explain. 4. If you have the same volume of two solutions of equal concentration, what do you know about the number of molecules? Steps involved in the preparation of a standard aqueous solution 10

11 Molarity Sample Problems 1. If 25.3 g of sodium carbonate is dissolved in enough water to make ml of solution, what is the concentration of sodium carbonate? 2. What mass of sodium carbonate is required to make 2.00 L of a 1.50 M solution? 11