In Stoichiometry PART I, we learned the basics about this kind of chemical bookkeeping. If
you have a balanced chemical equation, and some measured values, you can use this information
to find whatever is unknown.
It’s really common to measure the amount of one reactant, in order to figure out how
much product you expect to get. Or, given how much of one reactant you have, you might
want to figure out how much of a second reactant to add so you don’t have any left over.
Now, let’s move on to the more advanced problems - those that convert from
mass to mass.
It will take 3 steps: grams to moles, then moles to moles, and finally, moles to grams.
Let’s try it.
N2 + 3H2 → 2NH3
If you start with 15.0 grams of N2, and you have an excess of H2, how many grams of NH3
will you get? We need to first convert grams of N2 to moles
of N2. Then we can use our balanced chemical equation to go from moles of N2 to moles of
NH3. Finally, we’ll convert from moles of NH3 to grams of NH3.
Step 1: convert grams of N2 to moles of N2.
Remember, we're going to use conversion factors.
Start with what you know on the left, and what you want to end up with on the right.
We'll use the molar mass of N2 as a conversion factor.
We get the molar mass of N2 from the periodic table.
The molar mass of N2 = 2 times 14.01 = 28.02 grams/mole.
We’ll invert our conversion factor so the units cancel:
15.0 grams N2 ( 1 mole/28.02 grams) ...grams cancel...
... which gives us 0.54 moles N2.
Step 2: we use our balanced chemical equation to go from
number of moles of N2 to number of moles of NH3.
Start with what we know.
Leave room for a conversion factor...
..and end with what we want to find.
0.54 moles of N2 times a conversion factor will give us the number of moles of NH3.
The balanced chemical equation tells us for every 1 mole of N2, we get 2 moles of NH3.
Remember to put this conversion factor in the right orientation so the units cancel.
0.54 moles N2 times 2NH3 / 1 N2 = the number of moles of NH3
...moles of N2 cancels...
0.54 times 2 = 1.08 moles NH3.
Now finally, Step 3: we convert from moles of NH3 to grams of NH3.
We do that using the molar mass of NH3.
Start with what we know,and use a conversion factor to get to what we want in the end.
1.08 moles NH3 times the conversion factor = some number of grams of NH3.
Again, we get the molar mass from the Periodic Table. Molar
mass of NH3 = 14.01 + 3(1.008) = 17.03 grams/mole.
As always, make sure the conversion factor is in the right orientation so the units cancel.
1.08 moles NH3 times 17.03 g/mol
...Moles cancel...
...gives us 1.08 times 17.03 =18.4 grams of NH3.
Mole-to-mole and mass-to-mass are the two most common types of stoichiometry problems.
Problems involving gas volumes look a little different.
We’ll do those stoichiometry problems in another video.