Chapter 12 Stoichiometry

1. The part of chemistry that deals with the amount of substances involved in chemical reactions A. 3 basic steps to every stoichiometry problem -convert given to moles -use mole ratio to get from given moles to unknown moles -convert from moles unknown to whatever asked for B. use factor label method C. if you are given or asked to solve for moles, you can skip steps 1 or 3 depending on the problem

Mole Ratios A. the coefficients in a balanced equation tell us the ratio of particles that react or are produced in the reaction B. we can assume that these are moles C. we can use this ratio of moles to help us in conversions (we just compare the coefficients of whatever 2 substances we are asked to work with) D. since we will be working with the coefficients in the equation, the first thing we must do in all stoichiometry problems is to make sure the equation is balanced

Percent Yield A. The numbers we calculate in mathematical problems like mass-mass are theoretical values. When we actually do these reactions in the laboratory our results can be different. B. We need a way to compare these theoretical results to our actual lab results. We do this with percent yield calculations. C. percent yield = _____actual_____ X 100% theoretical

Limiting Reactants (in some reactions, not all of the reactants are always used “up”) A. in a chemical reaction, the limiting reactant is the substance that is completely consumed i. it is not present in sufficient amounts to react with all of the other reactant B. the reactants that are left, unused, are said to be in excess i. the amount of product is therefore determined by the limiting reactant C. sometimes in a stoichiometry calculation we are given 2 starting substances, we have to find out which one is the limiting reactant and use that in the calculation D. Solve the problem for each and use the one that produces less because it will be the limiting reactant