10/30/20131Fri, Nov 1• Lecture 14 (Connecting)– Finish examples from L13– Molar Mass and Percent Composition (8.5‐6)• Questions we’ll answer:– What information is communicated by the mole?– What is the mass of one mole of a substance?– What is mass percent of compounds and what use is it to us?Examples using NA1. How many atoms are there in 0.294 mol of sodium?2. How many moles are there in 4.97 x 10344.97 x 1024atoms of sulfur?3. How many moles are there in 78 g of silver?10/30/20132If we had 1 mol of bicycles…– How many mol bike saddles would we have?– Tires?– Spokes? (assuming 36 spokes/wheel)If we had 1 mol of isopropyl alcohol (C3H8O)…CCH3CH3HOH–How many mol O atoms would we have?– Carbon atoms?–Hydrogen atoms?Interpreting the Mole1 mol of table salt contains 6.022 x 1023NaCl formula units…this means:Table salt (NaCl).Interpreting the Mole10/30/20133Example. You have 3.0 mol of aluminum sulfate, Al2(SO4)3. Calculate the following:1. The moles of sulfate ions.2. The moles of oxygen atoms.3. The total number of Al2(SO4)3formula units.4. The total number of Al atoms.Molar Mass1.661 x 10‐24g? g/mol = 44.009 amu1 amuOne mole of an element or compound is a “handful” sized quantity. How much does one mole of a substance weigh?Find the molar mass of carbon dioxide, CO21 amu = 1.661 x 10–24g (1/12 the mass of one 12C atom) = 44.009 g/mol6.022 x 1023CO2molcs1 mol CO212.011 g/mol C15.999 g/mol O+ 244.009 g/mol CO2The mole and the amu are defined in such a manner that the atomic masses given in the periodic table can be interpreted as molar masses.12.011 amu/C 15.999 amu/O= 44.009 amu/CO2+ 210/30/201347How many grams of oxygen corresponds to 0.50 moles?? g O2= 0.5 mol O232 g O21 mol O2= 16 g O2Molar Mass as a Conversion FactorMolar mass (the mass of one mole of a substance), allows us to convert between macroscopic and particulate points of view.How many moles of oxygen are there in 1.2 grams?? mol O2=1.2 g O21 mol O232 g O2= 0.038 mol O2Examples1. Calculate the mass in grams of 4.85 mol of acetic acid, CH3COOH. 2. How many moles of ammonia (NH3) are there in 48.3 g?10/30/20135What’s in that compound, anyway?• The nomenclature conventions we’ve seen for binary compounds are designed so that the absolute ratios of the atoms in a chemical formula are directly communicated:– Carbon dioxide = CO2– Sodium chloride = NaCl– Iron(III) sulfide = Fe2S3• The chemical formulas of polyatomic ions, acids, and many other molecules must be memorized, but you still know the absolute ratios of the atoms:– Nitrate ion = NO3–– Sulfuric acid = H2SO4– Ammonia = NH3• But there was a time in the not‐too‐distant past when these chemical formulas were not known!• How do chemists figure out chemical formulas?Elemental AnalysisThe composition of unknown compounds can be determined by observing how the compound reacts to chemical or physical changes.Qualitative tests: • Sodium fusion test –halogens.• Schoeniger oxidation –Cl, N, SQuantitative tests:• Mass spectrometry –molecular mass• Gravimetry – ions (by selective precipitation)• Combustion analysis –C, H, N10/30/20136Percent Composition• Qualitative elemental analysis tells us what elements are present in a compound.• Quantitative elemental analysis tells us the masses of each element present in a sample of the compound.• The results of quantitative analyses of the elements in a compound are often presented in terms of percent composition:• In your book:% composition of mass of element X in sample of compound = 100of element X mass of sample of compound1% cmoompolesition of mass of element X in of compound = 100of element X mass of of compoun1m l d oeMore general…does not require knowledge of molar mass.Using Mass % to find Mass of ElementsExample. Pentane is a hydrocarbon, meaning it contains only carbon and hydrogen. If pentane is 83.23% carbon by mass, what percent of hydrogen is contained in pentane?How many grams of carbon and hydrogen are contained in a 15.86 g sample of pentane?10/30/20137Using Chemical Formula to Find Mass %Example. Determine the mass percent of each element in dihydrogen