The Mole Chapter 6

The Mole Remember it is a measurement for anything 1 mole means 6.022 x 1023 of anything This is called Avogadro’s Number A sample of an element with a mass equal to that element’s average atomic mass expressed in grams contains 1 mol of atoms 26.98 g of Aluminum has 6.022 x 1023 atoms = 1mol

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 have 0.00568 g of Silicon how many atoms do you have? Know that 1 mol Si is 28.09 g and 1 mole is 6.022 x 1023 atoms, these are your conversion factors So 0.00568 g x 1 mol/28.09 g = 0.000202 mol Si 0.000202 mol Si x 6.022 x 1023 atoms/ 1 mol = 1.22 x 1020 Si atoms

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 CH4 is made up of 1 mol of C and 4 mol of H 1 mol C = 1 x 12.01 g = 12.01 g 4 mol H = 4 x 1.008 g = 4.032 g So 1 mol of CH4 = 12.01 g + 4.032 g = 16.04 g Try to find molar mass of CaCO3 Answer = Now figure out how many grams of CaCO3 are in 4.86 mol

Practice Problem How many moles and how many molecules of C7H14O2 is in a bee sting if 1 gram is released? First find Molar Mass Answer Second find how many moles in 1 gram Finally change moles to molecules

Enough for 1 day, Watch the Video “The Mole”

Percent composition of Compounds Take mass of 1 element and divide it by the mass of entire compound and multiply by 100 Example: C2H5OH (ethanol) What is the mass percent of Carbon in this compound? What is mass of 1 mole of ethanol? Answer What is the mass percent of carbon in ethanol? Now do Hydrogen and Oxygen Answers

6.6 Formulas of Compounds Empirical Formula – simplest formula and expresses the smallest whole-number ratio of the atoms present C6H12O6 and C4H8O4 both have the same empirical formula CH2O Molecular Formula – the actual formula of a compound that tells you the composition of the molecules that are present C6H12O6 is the molecular formula for glucose

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

Example Problem An oxide of aluminum is formed by the reaction of 4.151 g of aluminum with 3.692 g of oxygen. What is the empirical formula of the compound formed? Relative Masses must be converted to moles 4.151 g Al x 1 mol Al/ 26.98 g Al = 0.1539 mol of Al atoms 3.692 g O x 1 mol O/ 16.oo g O= 0.2308 mol of O atoms Divide by smallest number of moles 0.1539 mol Al / 0.1539 = 1.000 mol Al 0.2308 mol O / 0.1539 = 1.500 mol O Multiply by smallest number to make them all whole 1.500 O x 2 = 3 O atoms 1.000 Al x 2 = 2 Al atoms So formula is Al2O3

Practice Problem In a lab experiment it was observed that 0.6884 g of lead combines with 0.2356 g of chlorine to form a binary compound. What is the empirical formula of this compound? Step 1 Relative Masses Step 2 Convert to moles Step 3 Divide by smallest number Step 4 Multiply by smallest number that makes them all whole numbers Rules apply no matter how many elements are in the compound

Calculation of Molecular Formulas Must know percent composition and the molar mass It’s 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

Example If Empirical formula is P2O5 and molar mass is 283.88 g, what is compound’s molecular formula? Know we have 2 moles P and 5 moles O 2 mol P = 2 x 30.97 g = 61.94 g 5 mol O = 5 x 16.00 g = 80.00 g 1 mol P2O5 = 141.94 g Know empirical formula x n = molecular formula and that the molecular formula = molar mass This tells us n = molar mass/ empirical formula So 283.88 g / 141.94 = n = 2 So (P2O5)2 means P4O10

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 98.96 g?