CHAPTER 6 CHEMICAL COMPOSITION Problems 1 13 17 31 33 58 79 84 105 108 115 6 3 COUNTING ATOMS BY THE GRAM Atomic Masses and Molar Masses If 98 93 of C atoms are carbon 12 and 1 07 of C atoms are carbon 13 then the weighted average atomic mass of carbon is calculated as follows 12 000000 amu 0 9893 13 003354 amu 0 0107 12 01 amu So how many carbon atoms are present in 12 01 grams of carbon This number was determined experimentally to be 6 022 1023 It was named Avogadro s number to honor Italian scientist Amedeo Avogadro who first proposed in 1811 that the volume of a gas depended only on the number not type of atoms or molecules present at a given temperature and pressure AVOGADRO S NUMBER NA 6 022 1023 4 sig figs How big is this If 6 022 1023 hydrogen atoms were laid side by side the total length would encircle the Earth about a million times The mass of 6 022 1023 Olympic shotput balls is about equal to the mass of the Earth CHEM 121 Tro Chapter 6 The volume of 6 022 1023 softballs is about equal to the volume of the Earth page 1 of 6 THE MOLE CONCEPT 1 mole abbreviated mol 6 022 1023 items Similar to 1 dozen 12 items 1 dozen doughnuts 12 doughnuts 1 mole of doughnuts 6 022 1023 doughnuts MOLE CALCULATIONS I a How many eggs are in 3 dozen eggs b How many eggs are in 3 moles of eggs c How many moles of C atoms are present in a sample of 1 25 1024 C atoms Thus the mass of 1 mole of C atoms is 12 01 g or 12 01 g mol 1 mole 6 022 1023 is the amount of atoms of any element that has a mass in grams equal to the mass of ONE atom in amu The atomic masses reported for each element in the Periodic Table give the atomic mass in amu and the molar mass in g mol Molar mass MM Mass in grams of 1 mole of any element or compound Example Use the Periodic Table to provide the molar mass of each element below a Al CHEM 121 Tro Chapter 6 b Si c Ar d Sn page 2 of 6 6 4 COUNTING MOLECULES BY THE GRAM Molar mass MM Mass in grams of 1 mole of any element or compound To obtain the molar mass of a compound multiply the molar mass of each element by the number of each present then add them all up Example Determine the molar mass of each of the following compounds a O2 2 molar mass of O 2 16 00 g mol 32 00 g mol b NaCl c CO2 d H2SO4 e Ca3 PO4 2 Do not worry about rounding molar mass to the correct number of sig figs Molar mass will rarely be the measurement that limits the number of sig figs for a calculation CONVERSIONS AMONG MASS NUMBER OF MOLES AND NUMBER OF PARTICLES Use the dimensional unit analysis method 1 Write the units of the final answer 2 Write the given information related to the answer 3 Determine the unit factors Avogadro s molar masses necessary to solve the problem using the given information Ex 1 How many moles of helium are in 5 00 g of helium CHEM 121 Tro Chapter 6 page 3 of 6 Ex 2 How many moles of neon are in 25 0 g of neon Ex 3 How many neon atoms are in 25 0 g of neon 6 5 CHEMICAL FORMULAS AS CONVERSION FACTORS Ex 1 How many moles of H2O are in 50 0 g of H2O Ex 2 How many H2O molecules are in 50 0 g of H2O Ex 3 How many H atoms are in 50 0 g of H2O CHEM 121 Tro Chapter 6 page 4 of 6 MOLAR VOLUME The volume occupied by 1 mole of any gas Avogadro s Law At the same temperature and pressure equal volumes of gases contain the same number of molecules Standard temperature and pressure STP T 0 C and P 1 00 atm At STP 1 mole of gas occupies 22 4 L 3 sig figs Write 2 unit factors using this information Mole Calculations with Molar Volume Ex 1 What mass of CO2 occupies a volume of 1 05 L at STP Ex 2 What is the volume occupied by 12 5 g of H2S gas at STP Ex 3 How many He atoms are present in 0 124 L of He gas at STP CHEM 121 Tro Chapter 6 page 5 of 6 6 7 MASS PERCENT COMPOSITION FROM A CHEMICAL FORMULA Mass Percent Composition or Mass Percent The mass of each element in a compound divided by the mass of the entire compound Mass Percent of Element Mass of Element 100 Total Mass of Compound Steps to determine percentage composition 1 Calculate the total mass of each individual element in the compound 2 Add up all the masses of each element to get the total mass of the compound 3 Divide the mass of each individual element with the total mass of compound Ex 1 What is the percent composition by mass of each element in H2O H O Ex 2 What is the percent composition by mass of each element in K2SO4 K S O Ex 3 What is the percent composition by mass of each element in Al2 CO3 3 Al C O CHEM 121 Tro Chapter 6 page 6 of 6