Theoretical Yield Example Problem - Chemistry Homework

This example problem demonstrates how to predict the amount of product produced from a given amount of reactants. Problem Given the reactionNa2S(aq) 2 AgNO3(aq) → Ag2S(s) 2 NaNO3(aq)How many grams of Ag2S will form when 3.94 g of AgNO3 and an excess of Na2S are reacted together? Solution The key to solve this type of problem is to find the mole ratio between the product and the reactant.Step 1 - Find the atomic weight of AgNO3 and Ag2S.From the periodic table:Atomic weight of Ag 107.87 gAtomic weight of N 14 gAtomic weight of O 16 gAtomic weight of S 32.01 gAtomic weight of AgNO3 (107.87 g) (14.01 g) 3(16.00 g)Atomic weight of AgNO3 107.87 g 14.01 g 48.00 gAtomic weight of AgNO3 169.88 gAtomic weight of Ag2S 2(107.87 g) 32.01 gAtomic weight of Ag2S 215.74 g 32.01 gAtomic weight of Ag2S 247.75 gStep 2 - Find mole ratio between product and reactantThe reaction formula gives the whole number of moles needed to complete and balance the reaction. For this reaction, two moles of AgNO3 is needed to produce one mole of Ag2S.The mole ratio then is 1 mol Ag2S/2 mol AgNO3Step 3 Find amount of product produced.The excess of Na2S means all of the 3.94 g of AgNO3 will be used to complete the reaction.grams Ag2S 3.94 g AgNO3 x 1 mol AgNO3/169.88 g AgNO3 x 1 mol Ag2 S/2 mol AgNO3 x 247.75 g Ag2S/1 mol Ag2SNote the units cancel out, leaving only grams Ag2Sgrams Ag2S 2.87 g Ag2S Answer 2.87 g of Ag2S will be produced from 3.94 g of AgNO3.