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 H2O a 1 5 3 4 b mole of propane needs how much oxygen 1 5 5 1 c 2 moles oxygen 2 1 2 2 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 1molC molC 10 gC 0 833molC 12 gC 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 32 gO2 0 833molO2 26 7 gO2 1molO2 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 moles of solute 3 Molarity M Liters of solution
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