Copyright © Houghton Mifflin Company. All rights reserved. 8 | 1 Atomic Masses Balanced equations tell us the relative numbers of molecules of reactants.

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Copyright © Houghton Mifflin Company. All rights reserved. 8 | 1 Atomic Masses Balanced equations tell us the relative numbers of molecules of reactants and products. C + O 2  CO 2 1 atom of C reacts with 1 molecule of O 2 to make 1 molecule of CO 2

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 2 Atomic Masses (cont.) If you want to know how many O 2 molecules you will need, or how many CO 2 molecules you can make, you will need to know how many C atoms are in the sample. You can either count the atoms (not possible) or weigh the carbon and calculate the number of atoms. You can do this if you know the mass of one atom of carbon. Atomic masses allow us to convert weights into numbers of atoms. The unit is the amu (atomic mass unit) 1 amu = 1.66 x g

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 3 Atomic Masses (cont.)

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 4 Atomic Masses (cont.) If our sample of carbon weighs 3.00 x amu, how many carbon atoms are present? Since our equation tells us that 1 C atom reacts with 1 O 2 molecule, if we have 2.50 x C atoms, how many molecules of O 2 will we need?

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 5 Moles The mass of 1.0 mole of an element is equal to its atomic mass in grams. One mole = x units. This number is called Avogadro’s number. One mole of carbon-12 atoms weighs 12.0 g and has 6.02 x atoms. One atom of carbon-12 weighs 12.0 amu.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 6 Compute the number of moles and the number of atoms in 10.0 g of Al. Use the periodic table to determine the mass of 1 mole of Al. Use this as a conversion factor for converting grams to moles.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 7 Use Avogadro’s Number to determine the number of atoms in 1 mole. Use this as a conversion factor for converting moles to atoms.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 8 Compute the number of moles and the mass in g of 2.23 x atoms of Al. Use Avogadro’s Number to determine the number of atoms in 1 mole. Use this as a conversion factor for converting atoms to moles.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 9 Use the periodic table to determine the mass of 1 mole of Al. Use this as a conversion factor for converting moles to grams.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 10 Molar Mass Molar mass: the mass in grams of one mole of a compound The relative weights of molecules can be calculated from atomic masses. water = H 2 O = 2(1.008 amu) amu = amu 1 mole of H 2 O will weigh g, therefore the molar mass of H 2 O is g. 1 mole of H 2 O will contain g of oxygen and 2.02 g of hydrogen.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 11 A 12 oz Pepsi contains approximately 20 g of sucrose, whose molecular formula is C 12 H 22 O 11. How many molecules of sucrose are present?

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 12 Percent Composition Percentage by mass of each element in a compound Can be determined from the formula of the compound. The percentages may not always total to 100% due to rounding.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 13 Determine the percent composition of each element in the compound C 2 H 5 OH. Determine the mass of each element in 1 mole of the compound. Determine the molar mass of the compound by adding the masses of the elements.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 14 Divide the mass of each element by the molar mass of the compound and multiply by 100%

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 15 Calculate the mass % of each element in Ca(NO 3 ) 2.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 16 Calculate the mass of N in grams present in 5g of glycine (C 2 H 5 O 2 N)

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 17 Empirical Formulas Empirical formula: the simplest, whole-number ratio of atoms in a molecule Molecular formula: a multiple of the empirical formula Examples: CompoundEmpirical FormulaMolecular Formula BenzeneCHC 6 H 6 Hydrogen peroxideHOH 2 O 2 WaterH 2 OH 2 O

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 18 Determine the empirical formula of benzopyrene, C 20 H 12 Find the greatest common factor (GCF) of the subscripts. Divide each subscript by the GCF to get the empirical formula. Empirical Formula =

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 19 Determining the empirical formula from grams Obtain the mass of each element in g. Determine the number of moles of each element. Divide the number of moles of each element by the smallest number of moles present. If a non-integer results, multiply all the numbers by the smallest number integer that converts all of the numbers into whole number integers.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 20 When g of Zn is heated in pure oxygen, the sample gains 0.8 g of oxygen in forming the corresponding oxide. Calculate its empirical formula.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 21 When 1 g of chromium metal is heated with chlorine gas, 3.045g of a chromium chloride salt results. Calculate the empirical formula of the salt.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 22 Determine the empirical formula of acetic anhydride if its percent composition is 47% carbon, 6.0% hydrogen, and 47% oxygen. Convert the percentages to grams by assuming you have 100 g of the compound. Convert the grams to moles. Divide each by the smallest number of moles.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 23 If any of the ratios is not a whole number, multiply all the ratios by a factor to make it a whole number. Use the ratios as the subscripts in the empirical formula.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 24 Calculate the empirical formula of a compound that contains 40% carbon, 6.7% hydrogen, and 53.3% oxygen.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 25 Molecular Formulas The molecular formula is a multiple of the empirical formula. To determine the molecular formula you must know the empirical formula and the molar mass of the compound.

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 26 Determine the molar mass of the empirical formula. Divide the given molar mass of the compound by the molar mass of the empirical formula and round to the nearest whole number. Multiply the empirical formula by the calculated factor to give the molecular formula. Determine the molecular formula of benzopyrene if it has a molar mass of 252 g and an empirical formula of C 5 H 3

Copyright © Houghton Mifflin Company. All rights reserved. 8 | 27 If a compound with an empirical formula CH 2 O has a molar mass of , what is its molecular formula?