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The Mole Chapter 6

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The Mole Remember it is a measurement for anything Remember it is a measurement for anything 1 mole means x of anything 1 mole means x of anything This is called Avogadros Number This is called Avogadros Number A sample of an element with a mass equal to that elements average atomic mass expressed in grams contains 1 mol of atoms A sample of an element with a mass equal to that elements average atomic mass expressed in grams contains 1 mol of atoms g of Aluminum has x atoms = 1mol g of Aluminum has x atoms = 1mol

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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 mass If you know how many grams make 1 mole, you can figure out how many moles you have in a given mass If you have g of Silicon how many atoms do you have? If you have g of Silicon how many atoms do you have? Know that 1 mol Si is g and 1 mole is x atoms, these are your conversion factors Know that 1 mol Si is g and 1 mole is x atoms, these are your conversion factors So g x 1 mol/28.09 g = mol Si So g x 1 mol/28.09 g = mol Si mol Si x x atoms/ 1 mol = 1.22 x Si atoms mol Si x x atoms/ 1 mol = 1.22 x Si atoms

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Molar Mass What is the mass of 1 mole of a compound? Simply add the mass of the elements moles to get the compounds molar mass What is the mass of 1 mole of a compound? Simply add the mass of the elements moles to get the compounds molar mass CH 4 is made up of 1 mol of C and 4 mol of H CH 4 is made up of 1 mol of C and 4 mol of H 1 mol C = 1 x g = g 1 mol C = 1 x g = g 4 mol H = 4 x g = g 4 mol H = 4 x g = g So 1 mol of CH 4 = g g = g So 1 mol of CH 4 = g g = g Try to find molar mass of CaCO 3 Try to find molar mass of CaCO 3 Answer = Answer = Now figure out how many grams of CaCO 3 are in 4.86 mol Now figure out how many grams of CaCO 3 are in 4.86 mol Answer = Answer =

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Practice Problem How many moles and how many molecules of C 7 H 14 O 2 is in a bee sting if 1 gram is released? How many moles and how many molecules of C 7 H 14 O 2 is in a bee sting if 1 gram is released? First find Molar Mass First find Molar Mass Answer Answer Second find how many moles in 1 gram Second find how many moles in 1 gram Answer Answer Finally change moles to molecules Finally change moles to molecules Answer Answer

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Enough for 1 day, Watch the Video The Mole

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Percent composition of Compounds Take mass of 1 element and divide it by the mass of entire compound and multiply by 100 Take mass of 1 element and divide it by the mass of entire compound and multiply by 100 Example: C 2 H 5 OH (ethanol) What is the mass percent of Carbon in this compound? Example: C 2 H 5 OH (ethanol) What is the mass percent of Carbon in this compound? What is mass of 1 mole of ethanol? What is mass of 1 mole of ethanol? Answer Answer What is the mass percent of carbon in ethanol? What is the mass percent of carbon in ethanol? Answer Answer Now do Hydrogen and Oxygen Now do Hydrogen and Oxygen Answers Answers

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6.6 Formulas of Compounds Empirical Formula – simplest formula and expresses the smallest whole-number ratio of the atoms present Empirical Formula – simplest formula and expresses the smallest whole-number ratio of the atoms present C 6 H 12 O 6 and C 4 H 8 O 4 both have the same empirical formula CH 2 O C 6 H 12 O 6 and C 4 H 8 O 4 both have the same empirical formula CH 2 O Molecular Formula – the actual formula of a compound that tells you the composition of the molecules that are present Molecular Formula – the actual formula of a compound that tells you the composition of the molecules that are present C 6 H 12 O 6 is the molecular formula for glucose C 6 H 12 O 6 is the molecular formula for glucose

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6.7 Calculation of Empirical Formulas Important to learn the chemical formula of a new compound Important to learn the chemical formula of a new compound First figure out relative masses First figure out relative masses Second convert masses to number of moles Second convert masses to number of moles Divide by the smallest number of moles found Divide by the smallest number of moles found Multiply the numbers from 3 rd step by smallest number that will make them all whole numbers, this is the empirical formula Multiply the numbers from 3 rd step by smallest number that will make them all whole numbers, this is the empirical formula

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Example Problem An 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? An 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 moles Relative Masses must be converted to moles g Al x 1 mol Al/ g Al = mol of Al atoms g Al x 1 mol Al/ g Al = mol of Al atoms g O x 1 mol O/ 16.oo g O= mol of O atoms g O x 1 mol O/ 16.oo g O= mol of O atoms Divide by smallest number of moles Divide by smallest number of moles mol Al / = mol Al mol Al / = mol Al mol O / = mol O mol O / = mol O Multiply by smallest number to make them all whole Multiply by smallest number to make them all whole O x 2 = 3 O atoms O x 2 = 3 O atoms Al x 2 = 2 Al atoms Al x 2 = 2 Al atoms So formula is Al 2 O 3 So formula is Al 2 O 3

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Practice Problem In 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? In 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 Masses Step 1 Relative Masses Step 2 Convert to moles Step 2 Convert to moles Step 3 Divide by smallest number Step 3 Divide by smallest number Step 4 Multiply by smallest number that makes them all whole numbers Step 4 Multiply by smallest number that makes them all whole numbers Rules apply no matter how many elements are in the compound Rules apply no matter how many elements are in the compound

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Calculation of Molecular Formulas Must know percent composition and the molar mass Must know percent composition and the molar mass Its always a multiple of the empirical formula (Empirical Formula) n = molecular formula or Empirical Formula x n = molecular formula Its always a multiple of the empirical formula (Empirical Formula) n = molecular formula or Empirical Formula x n = molecular formula Compare the empirical formula to molar mass Compare the empirical formula to molar mass

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Example If Empirical formula is P 2 O 5 and molar mass is g, what is compounds molecular formula? If Empirical formula is P 2 O 5 and molar mass is g, what is compounds molecular formula? Know we have 2 moles P and 5 moles O Know we have 2 moles P and 5 moles O 2 mol P = 2 x g = g 2 mol P = 2 x g = g 5 mol O = 5 x g = g 5 mol O = 5 x g = g 1 mol P 2 O 5 = g 1 mol P 2 O 5 = g Know empirical formula x n = molecular formula and that the molecular formula = molar mass Know empirical formula x n = molecular formula and that the molecular formula = molar mass This tells us n = molar mass/ empirical formula This tells us n = molar mass/ empirical formula So g / = n = 2 So g / = n = 2 So (P 2 O 5 ) 2 means P 4 O 10 So (P 2 O 5 ) 2 means P 4 O 10

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Practice Problem What 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? What 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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