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CHEM 101 LECTURE NOTES Monday, September 8, 2003 Fall 2003 Dr. Joy Heising S 540-550 Chapter 2 Stoichiometry– describes the quantitative among elements relationships 1. in compounds (composition stoichiometry) 2. in chemical changes (reaction stoichiometry) Dalton’ s Atomic Theory – summarized experimental observations and interpretations in the nature of atoms: 1. an element is composed of extremely small, indivisible particles called atoms 2. Atoms cannot be created, destroyed, or transformed into atoms of another element 3. compounds are formed when atoms of different elements combine with each other in small, wholenumber ratios. 4. the relative numbers and kinds of atoms are constant in a given compound. 1 CHEM 101 LECTURE NOTES Monday, September 8, 2003 Fall 2003 Dr. Joy Heising S 540-550 Chemical formula – the chemical composition of a substance. Some examples of chemical formulas for elements: Monoatomic elements – sodium (Na), copper (Cu), barium (Ba) Diatomic elements – N2, O2, F2, Cl2, Br2, I2, H2 More complex molecules – S8, P4 For compounds, the formula indicates 1. elements present 2. the ratio in which the atoms of the elements occur Some examples: HCl – 1 H atom, 1 Cl atom 1 chemical formula H2O -2 H atoms, 1 O atom 1 chemical formula NH3 -1 N atoms, 3 H atoms 1 chemical formula C3 H8 – 3 C atoms, 8 H atoms 1 chemical formula 2 CHEM 101 LECTURE NOTES Monday, September 8, 2003 Fall 2003 Dr. Joy Heising S 540-550 molecular compounds: (Table 2.2) H2O2 hydrogen peroxide NH3 ammonia CH3CH2OH methanol CO2 carbon dioxide Methane CH4 benzene C6H6 Ion – an atom or group of atoms that carries an electrical charge Anion – negatively charged ions (Cl-, O2-) Cation – positively charged ions (Na+, Ba2+) Ionic compounds – extended array of ions in which the total positive and negative charges are equal (Fig 2.7) Formula unit –the simplest whole number ratio of ions in the compound (NaCl) Is PO43- a molecule? No. polyatomic anion. Compound names and formulas (table 2-3) NaCl Sulfuric acid Molecule sodium chloride H2SO4 ? NH4NO3 Na2SO4 ion 3 ammonium nitrate sodium sulfate CHEM 101 LECTURE NOTES Monday, September 8, 2003 Fall 2003 Dr. Joy Heising S 540-550 How much does a single atom weigh? Different elements weigh different amounts – related to what makes them unique. What units do we use to define the weight of an atom? amu – units of atomic weight. (atomic mass unit) 1 amu – 1/12 the mass of 1 “ carbon-12” atom. H Mg 1 amu 24.3 amu N Ag 14 amu 107.8 amu When scientists do experiments with substances (elements, molecules, ionic compounds), they usually - Use the chemical formula to plan (ratio of atoms, NaCl) - Measure the amount needed in grams How can we relate grams to the chemical formula? Mole – 6.022 x 1023 atoms, molecules or formula units Avogadro’ s number **one mole of atoms of an element has a mass in grams numerically equal to the atomic weight of the element** 1 H atom 1 mol H 1 amu 1.01 g 1 C atom 1 mol C 12 amu 12.01 g 4 CHEM 101 LECTURE NOTES Monday, September 8, 2003 Fall 2003 Dr. Joy Heising S 540-550 Why do we use such a weird number? Because it gives us an ‘ easy’ relationship between amus and grams. How many atoms are contained in 1.67 moles of Mg? How many grams does 1.67 moles of Mg weigh? = 1.00 x 1024 atoms = 40.6 grams The ratio of atoms in the simplest formula for a compound is the same as the ratio of moles of atoms of the elements in a sample of the compound 1 H2O molecule 2 H atoms 1 O atom 1 mol H2O 2 mol H 1 mol O How many molecules are there in one mole of propane? 6.022 x 1023 molecules In one mole of chlorine (Cl2), what is the number of a) Cl2 molecules? 6.022 x 1023 molecules b) Cl atoms? 2(6.022 x 1023 molecules) 5 CHEM 101 LECTURE NOTES Monday, September 8, 2003 Fall 2003 Dr. Joy Heising S 540-550 In one mole of Calcium nitrate, what is the number of a) formula units? 6.022 x 1023 b) Ca2+ cations? 6.022 x 1023 c) Nitrate anions? 2(6.022 x 1023) How much does H2O weigh? Formula weights – the sum of the atomic weights of the atoms in the formula. Propane C3 H8 3(12.01) + 8(1.00) = 44.03 Chlorine Cl2 2(35.45) = 70.90 Calcium nitrate Ca(NO3)2 40.08+ 2[14.01 + 3(16.00)] = 164.10 formula weight – all compounds, molecules and elements atomic weight – atoms molecular weight – molecules Molar mass –the mass of one mole of a substance. (g/mol) Numerically equivalent to formula weight (amu/formula unit) 6 CHEM 101 LECTURE NOTES Monday, September 8, 2003 Fall 2003 Dr. Joy Heising S 540-550 Calculate the number of C3H8 molecules in 74.6 g propane. 7 CHEM 101 LECTURE NOTES Monday, September 8, 2003 Fall 2003 Dr. Joy Heising S 540-550 Percent composition –the % of each element in the compound (by mass) ex. calculate the percent composition of C and H in C3H8. Molar mass of C3H8 = C H 3 (12.01 g/mol) 8 (1.008 g/mol) % carbon: = 36.03 (g/mol) 44.09 (g/mol) x % hydrogen = 8.064 (g/mol) 44.09 (g/mol) x = = 36.03 (g/mol) 8.064 (g/mol) 44.09 (g/mol) 100 = 81.71% 100 = 18.29% empirical formula –the smallest, whole number ratio of elements present NaCl H2 O H2O2 - HO Molecular formula –actual number of atoms present in a molecule 8 CHEM 101 LECTURE NOTES Monday, September 8, 2003 Fall 2003 Dr. Joy Heising S 540-550 If we have the % composition of a substance, we can determine the empirical formula Ex. 24.74% K 24.74 g K 34.76% Mn 34.76 g Mn 40.50% O 40.50 g O 100.00% 100 g substance 24.74 g K x 1 mol K 39.10 g = 0.6327 mol K 34.76 g Mn x 1 mol Mn 54.94 g = 0.6327 mol Mn 40.50 g O x 1 mol O 16.00 g = 2.531 mol O divide through by smallest # moles to get the ratio: 0.6327 mol K/0.6327 0.6327 mol Mn/0.6327 2.532 mol O/0.6327 =1 =1 =4 empirical formula is KMnO4 but we still don’ t know the molecular formula unless we also have the molar mass of the substance ex. in previous example, if molar mass = 158 g/mol empirical formula = molecular formula 9 CHEM 101 LECTURE NOTES Monday, September 8, 2003 Fall 2003 Dr. Joy Heising S 540-550 if a compound has the empirical formula HO and its molar mass = 34.0 g/mol HO = 1.0 + 16.0 = 17.0 g/mol 34/17 =2 2(HO) = H2O2 molecular formula Chapter 2: Composition stoichiometry – the relative ratios of different elements within one particular compound or molecule Chapter 3: Reaction stoichiometry – the relative ratios between different substances as they react with each other 10