Untitled - Kelly Walsh High School

42 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
Now that we pulled all the information from the problem, we no longer need
the original problem.
Having the pertinent information, we can go to the second step that involves
the equation.
In order to work any problem dealing with a chemical reaction, you must have a
balanced chemical equation.
Since our initial equation is not balanced, we need to balance it.
I2(s) F2(g) l 2 IF5(g) 75.0 g 4.00 1023 molecules 45.2 g
? percent yield
The most important concept when working stoichiometry problems such as this
one is moles. We must have moles to proceed. The mole determination of
iodine will involve the molar mass of iodine (2 126.9 g/mol), while the mole
determination of fluorine will involve Avogadro’s number (since we have number
of fluorine molecules). We can find the moles of each as follows:
mol I2 (75.0 g I2 )a 1 mol I2 b 0.295508 mol I2 (unrounded)
253.8 g I2 mol F2 (4.00 1023 1 mol F2 molecules F2 )a
6.022 1023 b
molecules F2 0.66423 mol F 2 (unrounded)
We could determine the moles of IF 5 at this point; however, we will forego that
pleasure until we finish with the reactants.
We now have the moles of the two reactants. This should ring a bell–any time
you have the quantity of more than one reactant, you need to know which is the
limiting reagent. For this reason, our next step will be to determine which of our
reactants limits the reaction. We can determine which reactant is the limiting
reagent by dividing each of the moles by the coefficient of each reactant from