Notes Sept 19 2013 ConcepTest For the reaction a N2 b O2 N2aO2b If 2 8 g of N2 yields 9 2 g of N2aO2b what are a and b respectively A 1 1 B 1 2 C 2 1 D 2 2 N2 2 8 g O2 6 4 g E need more info NO 9 2 g Mass Relationships in Reactions Law of Conservation of Matter Mass of reactants equals mass of products no change in amounts of constituent atoms in reaaction Balanced chemical equations must always include same number of each kind of atom on both sides of equation ConcepTest For the reaction 7 09 g of chlorine gas A 3 64 g Cl2 g H2 g 2 HCl g How many grams of HCl are produced from B 7 29 g C 7 09 g D 7 09 g E none of these ConcepTest 4 NH3 5 O2 4 NO 6 H2O If 17 g of ammonia reacts how many moles of hydrogen atoms are contained in the products A 1 mol B 6 mol C 1 5 mol D 0 66 mol E none of these 17 g NH3 x x 2 99 mol H Calculation Srategy 1 Mass to moles use formula weight 2 Ratio to moles use stoichiometric coefficients 3 Moles to mass use formula weight ConcepTest Zn 2 HCl ZnCl2 H2 If 65 4 g of Zn react with 36 5 g of HCl what is the total mass of ZnCl2 and H2 A 101 9 B 69 2 C 138 4 D 50 5 E none of these Only half the Zn is used because it needs the ratio 1 2 Limiting Reagent build a toy recipes ConcepTest Consider the reaction Zn 2 HCl ZnCl2 H2 Which case has zinc as the limiting reagent A 1 0 mol Zn 1 0 mol HCl C 0 5 mol Zn 2 0 mol HCl B 1 5 mol Zn 2 0 mol HCl D 2 0 mol Zn 1 0 mol HCl E 1 0 mol Zn 2 0 mol HCl 5 1 5 10 ConcepTest Consider the reaction C3H8 g 5 O2 g 3 CO2 g 4 H2O l If 8 80 g of propane react with 38 8 g of oxygen what reagent remains unused and in what amount A 4 4 g of propane ConcepTest What mass of silver chloride AgCl can Info for Stoichiometry 1 Formulas 2 Ratios of Reagents Most comes from experiments Quantitative chemical analysis AgNO3 aq NaCl aq AgCl s NaNO3 Analysis and Chemical Equations Quantitative Analysis use known reaction known amounts of one substance to determine another substance Combustion Analysis burn known amount and analyze carbon hydrogen and other atoms CxHy g H2O mol H2O Mol H mol C empirical formula gCO2 ConcepTest Which empirical formula corresponds to a molecule that has the formula weight of 88 A CH B CH2 C CH2O D C2H4OE none of these