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3 Stoichiometry Solutions to Exercises (c) Combining ratios are molecule and mole ratios. Since different molecules have different masses, equal masses of different reactants will not have equal numbers of molecules. By comparing initial moles, we compare numbers of available reactant molecules, the fundamental combining units in a chemical reaction. 3.72 (a) Theoretical yield is the maximum amount of product possible, as predicted by stoichiometry; assuming that the limiting reactant is converted entirely to product. Actual yield is the amount of product actually obtained, less than or equal to the theoretical yield. Percent yield is the ratio of (actual yield to theoretical yield) X 100. (b) No reaction is perfect. Not all reactant molecules come together effectively to form products; alternative reaction pathways may produce secondary products and reduce the amount of desired product actually obtained, or it might not be possible to completely isolate the desired product from the reaction mixture. In any case, these factors reduce the actual yield of a reaction. (c) No, 110% actual yield is not possible. Theoretical yield is the maximum possible amount of pure product, assuming all available limiting reactant is converted to product, and that all product is isolated. If an actual yield of 110% if obtained, the product must contain impurities which increase the experimental mass. 3.73 (a) Each bicycle needs 2 wheels, 1 frame, and 1 set of handlebars. A total of 4815 wheels corresponds to 2407.5 pairs of wheels. This is more than the number of frames or handlebars. The 2255 handlebars determine that 2255 bicycles can be produced. (b) 2305 frames - 2255 bicycles = 50 frames left over 2407.5 pairs of wheels - 2255 bicycles = 152.5 pairs of wheels left over 2(152.5) = 305 wheels left over (c) The handlebars are the "limiting reactant" in that they determine the number of bicycles that can be produced. 1 bottle 3.74 (a) 48,775 L beverage X = 137,394.37 = X portions of beverage 0.355 L (The uncertainty in 355 mL limits the precision of the number of portions we can reasonably expect to deliver to three significant figures.) 126,515 bottles; 108,500 caps; 1.09 X 10⁵ bottles can be filled and capped. (b) 126,515 empty bottles - 108,500 caps = 18,015 bottles remain 137,394 portions - 108,500 caps =28,894 = 2.89 10⁴ portions remain (Uncertainty in the number of portions available limits the results to 3 sig fig.) (c) The caps limit production. 62