2 The Mole Remember it is a measurement for anything 1 mole means x 1023 of anythingThis is called Avogadro’s NumberA sample of an element with a mass equal to that element’s average atomic mass expressed in grams contains 1 mol of atoms26.98 g of Aluminum has x 1023 atoms = 1mol
3 How can this help us?If you know how many grams make 1 mole, you can figure out how many moles you have in a given massIf you have g of Silicon how many atoms do you have?Know that 1 mol Si is g and 1 mole is x 1023 atoms, these are your conversion factorsSo g x 1 mol/28.09 g = mol Simol Si x x 1023 atoms/ 1 mol = 1.22 x 1020 Si atoms
4 Molar MassWhat is the mass of 1 mole of a compound? Simply add the mass of the elements moles to get the compounds molar massCH4 is made up of 1 mol of C and 4 mol of H1 mol C = 1 x g = g4 mol H = 4 x g = gSo 1 mol of CH4 = g g = gTry to find molar mass of CaCO3Answer =Now figure out how many grams of CaCO3 are in 4.86 mol
5 Practice ProblemHow many moles and how many molecules of C7H14O2 is in a bee sting if 1 gram is released?First find Molar MassAnswerSecond find how many moles in 1 gramFinally change moles to molecules
7 Percent composition of Compounds Take mass of 1 element and divide it by the mass of entire compound and multiply by 100Example: C2H5OH (ethanol) What is the mass percent of Carbon in this compound?What is mass of 1 mole of ethanol?AnswerWhat is the mass percent of carbon in ethanol?Now do Hydrogen and OxygenAnswers
8 6.6 Formulas of CompoundsEmpirical Formula – simplest formula and expresses the smallest whole-number ratio of the atoms presentC6H12O6 and C4H8O4 both have the same empirical formula CH2OMolecular Formula – the actual formula of a compound that tells you the composition of the molecules that are presentC6H12O6 is the molecular formula for glucose
9 6.7 Calculation of Empirical Formulas Important to learn the chemical formula of a new compoundFirst figure out relative massesSecond convert masses to number of molesDivide by the smallest number of moles foundMultiply the numbers from 3rd step by smallest number that will make them all whole numbers, this is the empirical formula
10 Example ProblemAn oxide of aluminum is formed by the reaction of g of aluminum with g of oxygen. What is the empirical formula of the compound formed?Relative Masses must be converted to moles4.151 g Al x 1 mol Al/ g Al = mol of Al atoms3.692 g O x 1 mol O/ 16.oo g O= mol of O atomsDivide by smallest number of molesmol Al / = mol Almol O / = mol OMultiply by smallest number to make them all whole1.500 O x 2 = 3 O atoms1.000 Al x 2 = 2 Al atomsSo formula is Al2O3
11 Practice ProblemIn a lab experiment it was observed that g of lead combines with g of chlorine to form a binary compound. What is the empirical formula of this compound?Step 1 Relative MassesStep 2 Convert to molesStep 3 Divide by smallest numberStep 4 Multiply by smallest number that makes them all whole numbersRules apply no matter how many elements are in the compound
12 Calculation of Molecular Formulas Must know percent composition and the molar massIt’s always a multiple of the empirical formula (Empirical Formula)n = molecular formula or Empirical Formula x n = molecular formulaCompare the empirical formula to molar mass
13 ExampleIf Empirical formula is P2O5 and molar mass is g, what is compound’s molecular formula?Know we have 2 moles P and 5 moles O2 mol P = 2 x g = g5 mol O = 5 x g = g1 mol P2O5 = gKnow empirical formula x n = molecular formula and that the molecular formula = molar massThis tells us n = molar mass/ empirical formulaSo g / = n = 2So (P2O5)2 means P4O10
14 Practice ProblemWhat are the empirical and molecular formulas of a compound with these percent compositions 71.65% Cl, 24.27% C, 4.07% H and a molar mass of g?
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