Chapter 1 Stoichiometry Stoichiometry is the quantitative relationship between the elements in a compound or between the compounds elements in a chemical reaction Please review Appendix A2 A4 Significant Figures Reporting Quantitative Measurements and Results 1 Section 1 0 Mass of an Atom and Moles of Atoms Look at a periodic table For single atoms amu atomic mass unit 1 66 X 10 24 g For moles of atoms 6 022 x 1023 atoms the unit is grams H atom weighs 1 0079 amu or 1 67 x 10 24 g Mole of H atoms weighs 1 0079 g 2 Section 1 1 A Ca NO3 2 sample contains 2 04 grams of nitrogen How much does the Ca NO3 2 sample weigh Strategy to Solve grams N moles N moles Ca NO3 2 grams Ca NO3 2 ratio ratio ratio Ratios are chosen such that 3 Section 1 1 Ratios from Periodic Table 40 078 g Ca 1 mol Ca 14 007 g N 1 mol N 15 999 g O 1 mol O Ratios from Chemical Formula Ca NO3 2 1 mol Ca 1 mol Ca NO3 2 1 mol Ca 2 mol N Section 1 1 2 mol N 1 mol Ca NO3 2 1 mol Ca 6 mol O 164 086 g Ca NO3 2 1 mol Ca NO3 2 6 mol O 1 mol Ca NO3 2 2 mol N 6 mol O 4 A Ca NO3 2 sample contains 2 04 grams of nitrogen How much does the Ca NO3 2 sample weigh grams N moles N moles Ca NO3 2 grams Ca NO3 2 ratio ratio ratio 5 Section 1 1 Elemental Composition Mass Percent A mass A total mass 100 H2O2 hydrogen peroxide Molecular Weight 2 1 0079 g mol H 2 15 999 g mol O 34 0138 g mol H2O2 H O 6 Section 1 2 Empirical Formula the simplest formula only tells you the ratio of elements in a compound A 7 50 g sample of iron is heated in oxygen to form an iron oxide If 10 36 g of the oxide is formed what is the empirical formula of this oxide of iron We need the ratio moles Fe moles O 7 Section 1 2 Empirical Formula Hint divide larger by smaller Now try multiples that lead to whole numbers This is the simplest formula of this compound Molecular Formula the actual number of atoms of each element in a compound For example the empirical formula of benzene is CH CH does not exist the molecular formula of benzene 8 is C H 6 6 Section 1 2 Empirical Formula by Combustion If 12 30 g of a white substance was burned to give 7 40 grams of water and 18 04 grams of carbon dioxide What is the empirical formula of the substance CxHyOz CxHyOz O2 X CO2 Y 2 H2O 18 04 g 7 40 g 12 30 g Figure out mole C in the original sample solve for X Figure out mole H in the original sample solve for Y 9 Section 1 2 Empirical Formula by CxHyOzCombustion O2 X CO2 Y 2 H2O 12 30 g 18 04 g 7 40 g Figure out how many grams of O had to be in the original Sample and solve for Z 10 Section 1 2 Mole Ratio Need whole numbers so divide each by the smallest Empirical Formula the simplest formula only tells you the ratio of elements in a compound 11 Section 1 2 Molecular Formula We need the molecular weight to know the molecular formula The molecular weight can be determined by other techniques presented next week Given an empirical formula of CH2O Molecular WeightChemical Formula 30 g mol CH2O Chemical Formula or Molecular Formula the actual number of atoms of each element in a compound 12 Section 1 2
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