ES 105 2006 January 26 Molar quantities of substances I. Molar mass of substance is numerically equal to the formula mass A. Molar mass in grams B. Formula mass in ‘u’—atomic mass units 1. Examples a. C + O2-->CO2 1) 1 mole C = 12 g 2) 1 mole O2 = 32 g 3) 1 mole CO2 = 44 g b. NO + O2+ --> NO2 In class activity 1) Balance equation 2) Determine formula mass of each molecule 3) Find molar mass ratios using balanced formula and masses C. Can calculate mass of a number of moles two ways 1. use molar mass ratios a. first find formula mass of each substance b. then use BALANCED chemical equation to find molar ratios c. use proportions to compare mass of one substance to another 1) IN THE MOLAR RATIOS FROM THE CHEMICAL EQUATION 2) SOLVE PROPORTIONS WITH MULT PROP OF EQUAL!! 2. or convert mass to moles of substance given a. use molar ratios to determine moles of other substance b. convert those moles back to grams of that substance 3. Examples a. C + O2-->CO2 1) Molar mass ratio 12:32:44 reduces to 3:8:11 2) So, if you have 4 grams of oxygen, a) how many grams of carbon will be consumed? b) How many grams of carbon dioxide will be produced? b. NO + O2+ --> NO2 In class activity 1) If you have 120 grams of NO to react, how many grams of NO2 are produced? c. H2S + O2 --> SO2 + H2O 1) Molar mass ratios? 2) If you get 16 g of SO2, how much O2 was used? II. Molar relationships in Chemical Equations A. Shown by coefficients in balanced equations B. Called STOICHIOMETRIC FACTORS 1. propane C3H8 + 5 O2 --> 3 CO2 + 4 H2Oa. 1:5:3:4 b. ½ mole of propane needs how much oxygen? c. 212251521==⋅ moles oxygen C. Mass relationships in chemical equations 1. equation tells molar ratios a. calculate formula mass of each reactant and product b. find moles of substances from mass c. calculate moles of reactants from ratios d. convert back to grams from formula mass 2. Examples a. C + O2-->CO2 How much oxygen is needed to react with 10 g carbon? 1) Formula mass of molecules a) 1 mole C = 12 g b) 1 mole O2 = 32 g c) 1 mole CO2 = 44 g d) Ratio 3:8:11 2) 10 g carbon = ? moles molCgCmolCgCmolC 833.012110? =⋅= 3) 0.833 moles of carbon needs how many moles oxygen a) Stoichiometric ratio is 1:1 b) 0.833 moles of oxygen 4) 0.833 moles of oxygen has what mass? 22227.26132833.0 gOmolOgOmolO =⋅ b. In class activity Calculate the mass of oxygen needed to react with 3.5 moles of nitrogen to form nitrogen dioxide 1) N2 + O2 --> NO2 a) calculate formula mass of each reactant and product b) find moles of substances from mass c) calculate moles of reactants from ratios d) convert back to grams from formula mass 2) or use stoichiometric ratios on the masses directly III. Solutions A. Mixture of two or more substances 1. homogeneous evenly and completely mixed 2. two parts: solvent and solute a. there is a limit of amount of solute that a solvent can hold b. dilute vs. concentrated c. water is common solvent 3. Molarity (M)
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