Problem solving pattern Stoichiometry Problem solving pattern

Priority 1 Is the equation properly balanced? Several thoughts here Do you need to determine any reactants or products? If so, do so. If not, move on to check if properly balanced Law of Conservation of Matter: number of atoms of each element on both sides should be equal, so…count! Don’t forget that you cannot change subscripts, only coefficients.

Priority 2 Since this is a stoichiometry problem, we already know that we are determining some information based on the chemical reaction we are given. Whether we are given moles, grams, liters, or a concentration (moles/liter) of a given reactant or product, we will need to convert this into moles for whatever determination we need to make. You may need to calculate molar mass, or convert from liters to moles (22.4L/mol) for this step. If a concentration and volume given, note that (Mol/L)(L) = moles.

Continuing… Since this is a stoich problem, we know we will be using the ratios of the coefficients to arrive at our destination. Using our dimensional analysis format (“gift tag” conversion), we will be able to determine the number of moles of that reactant or product we are searching for. Example: CH4 + 3O2  CO2 + 2H2O we “see” the invisible ones in front of the methane and carbon dioxide If we have 3 moles of methane, how many moles of water will be formed when it is totally combusted? Using “gift tag,” (3 mol CH4)( 2 mol H2O/1 mol CH4) = 6 mol H2O formed Note that the mol CH4 cancel out and we are left with mol H2O

Continuing… One final step may be necessary…do you need to convert from moles to grams, liters, or for you final answer? You, again, may need the appropriate molar mass or gas/mole conversion (22.4L/mol) to get where you need to go

Because I want you to be successful… Set up all your equations and make sure you end up with the units you are looking for at the end of each one…if not, don’t bother picking up the calculator, as the answer will not be useful. Look at your numbers, prior to using calculator…think about where the number should be. Do this on all calculations…it will keep you from putting down some weird stuff if you miss-hit a key…saw this too many times recently, in a variety of places. Always estimate.