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Chapter Two: ATOMS, MOLECULES, AND IONS Performance goal • Students should be able to: • Show how the postulates of the atomic theory “ explain” the laws of conservation of Mass, Constant Composition (definite proportions) and multiple proportions. • Describe the atomic structure • Given the composition of a nucleus, give the atomic and mass numbers. Perform the reverse calculation • Distinguish between atom, molecule and ion. Give example of each. • Understand and know how to use the periodic table • Name simple compounds Copyright © Houghton Mifflin Company. All rights reserved. 2–2 2.2 Fundamental Chemical Laws Law of conservation of mass Antoine Lavoisier (1743-1794) by carefully weighing the reactants and products of various reactions made the following suggestion • Mass cannot be created nor destroyed. Show Law of conservation video of mercury (II) oxide http://www.brookscole.com/cgi-wadsworth/course_products_wp.pl?fid=M20b&product_isbn_issn=0495012130&discipline_number=12 Copyright © Houghton Mifflin Company. All rights reserved. 2–4 • You have a chemical in a sealed glass container filled with air. The setup is sitting on a balance as shown below. The chemical is ignited by means of a magnifying glass focusing sunlight on the reactant. After the chemical has completely burned, which of the following is true? Explain your answer. – A. the balance will read less than 250.0g. – B. The balance will read 250.0g. – C. The balance will read greater than 250.g – D. Cannot be determined without knowing the identity of the chemical. Copyright © Houghton Mifflin Company. All rights reserved. 2–5 Learning Check • A 0.406 g sample of magnesium reacts with oxygen, producing 0.674 g magnesium oxide as the only product. What mass of oxygen was consumed in the reaction? Copyright © Houghton Mifflin Company. All rights reserved. 2–6 Law of definite proportion Joseph Proust (1754-1826), proposed the constant composition of compounds that states • A given compound always contains exactly the same proportion of elements by mass Copyright © Houghton Mifflin Company. All rights reserved. 2–7 Example of the Law Definite Proportions • Let’s consider water Sample A weighed 10.000 g 1.119 g H 8.881 g O Sample B weighed 27.000 g 3.021 g H 23.979 g O Composition is always % H = 11.19 % O = 88.81 Copyright © Houghton Mifflin Company. All rights reserved. 2–8 Learning Check • 3.06 g of hydrogen react with oxygen where 27.35 g of water are obtained. In a second experiment, water is decomposed resulting in 1.45 g of hydrogen & 11.51 g of oxygen. Is this consistent with the law of definite proportions? Copyright © Houghton Mifflin Company. All rights reserved. 2–9 Law of multiple proportions John Dalton was a strong proponent of the Law of Multiple Proportions (which is the 3rd postulate of Dalton’s atomic theory. • When two elements form a series of compounds, the ratios of the masses of the second element that combine with a fixed mass (ex. 1 gram) of the first element can always be reduced to small whole numbers. Copyright © Houghton Mifflin Company. All rights reserved. 2–10 Example of the Law of Multiple Proportions • That of nitrogen and oxygen forming three different compounds that contain different relative amounts of nitrogen and oxygen. Compound A Compound B Compound C Mass of Nitrogen that combines with 1 g of Oxygen 1.750 g 0.8750 g 0.4375 g Copyright © Houghton Mifflin Company. All rights reserved. 2–11 Example of the Law of Multiple Proportions continued The ratios of the masses of nitrogen combining with 1 gram of oxygen in each pair of compound should be small whole integer number A = 1.750 = 2 for nitrogen 0.8750 B 0.8750 = 1 for nitrogen, a ratio 2:1 0.8750 B = 0.8750 = 2 for nitrogen 0.4375 C 0.4375 = 1 for nitrogen, a ratio 2:1 0.4375 A = 1.750 = 4 for nitrogen 0.4375 C 0.4375 = 1 for nitrogen, a ratio 4:1 0.4375 Copyright © Houghton Mifflin Company. All rights reserved. 2–12 Which of the following pairs of compounds can be used to illustrate the law of multiple proportions? • • • • • • CH4 and CO2 NH4 and NH4Cl NO and NO2 H2O and HCl ZnO2 and ZnCl2 CO and CO2 Copyright © Houghton Mifflin Company. All rights reserved. 2–13 Which of the following pairs of compounds can be used to illustrate the law of multiple proportions? • • • • • • CH4 and CO2 NH4 and NH4Cl NO and NO2 H2O and HCl ZnO2 and ZnCl2 CO and CO2 Copyright © Houghton Mifflin Company. All rights reserved. 2–14 QUESTION Two common air pollutants when analyzed yield the following information; Compound I shows that 1 gram of sulfur combines with 1 gram of oxygen. Compound II shows that 1.5 grams of oxygen combines with 1 gram of sulfur. This is consistent with the law of multiple proportions because… 1. the analysis shows that sulfur and oxygen can combine in more than one way. 2. the analysis shows that the combination of sulfur and oxygen is very close to 1 in both cases. 3. the analysis shows that a small whole number (2) could be used to make a ratio of 2:3 for the oxygen mass in compound I compared to compound II. Copyright © Houghton Mifflin Company. All rights reserved. 2–15 ANSWER Choice 3 offers the same conclusion as the actual Law of Multiple Proportions. If 1 and 1.5 are both multiplied by 2 the ratio is still maintained (1.5 to 1 is the same as 3:2 so multiplying by 2 does not change the ratio and does produce whole numbers for both). Copyright © Houghton Mifflin Company. All rights reserved. 2–16 Learning Check • Two different compounds are formed by the elements carbon & oxygen. The first compound contains 42.9% by mass carbon and 57.1% by mass oxygen. The second compound contains 27.3% by mass carbon and 72.7% by mass oxygen. Show that the data are consistent with the Law of Multiple Proportions. (assume 100 g is in each compound) Copyright © Houghton Mifflin Company. All rights reserved. 2–17 Show your work Copyright © Houghton Mifflin Company. All rights reserved. 2–18