Stoichiometry!- Intro and Beginning Problems

I took Chemistry in tenth grade and I remember that at the beginning of the semester, he said "Eventually, stoichiometry will become a cuss word and haunt you in your dreams."  How right he was.  By the middle of the semester, just about every problem we had to solve had something to do with stoichiometry.  Eventually, stoichiometry was everywhere.  It was in my Algebra II class, my English II class, and even in Theater II.  Any word that ended in "-ometry" made me want to run in the opposite direction and just keep on running.  I finally was able to suck up some courage to fight the beast.  I walked into that classroom, sword in hand, and slayed it.  I got a few scratches while I was at it, but I made it out alive.  I started to see the lighter side of stoichiometry and no longer feared Chemistry.  I started to get braver and braver to the point that I chose Chemistry as my major in college.  To help those from getting those scratches that I had to get, I decided to be the shield and not only major in Chemistry, but to teach the subject.  I'm still working on my degree, but it doesn't mean that I still can't teach a few things here and there.  After all, it is a known fact that if you struggle with something, try teaching it to someone.  By walking them through it, it helps you to understand it a little better (especially if it's something that you're good at and just need to enhance it a bit more.)

So.  Stoichiometry.  Yeah.  What is it?  By definition, stoichiometry (stoy-key-om-eh-tree) is the calculation of quantities of chemical elements or compounds involved in a chemical reaction (dictionary.com).  In simpler terms, how much of something in a reaction.  An example of stoichiometry is:  A cup contains 30.2 grams of water.  How many moles of water is that? and How many grams of sodium chloride are needed to make  200 mL of a 0.5 M solution?  Problems can be more complicated that these two, but for now, I'll just start with easier ones.

Now, how exactly are these solved?  The same way how you eat an elephant: one bite at a time.  So let's begin by separating the two problems to where it is easier on the eyes.

a) A cup contains 30.2 g of water.  How many moles of water is that?

b) How many grams of sodium chloride are needed to make a 200 mL of a 0.5 M solution?

Good.  Since the spare words in the paragraph are gone, it's much easier on the eyes to find the problem and aren't anywhere near as intimidating as they looked with the long paragraph.  So...now what?  Let's take these one problem at a time.  We can easily make problem a more friendly by just taking out of it what we need.  By doing this, it becomes:

a) 30.2g H2O  moles of H2O = ?

That's friendlier now that all the useless other words are out of the problem.  But now what?  Well, to finish this, the periodic table is needed.  (Yes, we're doing chemistry that involves using the periodic table.  Imagine that!)  Using the periodic table, you need to find the molecular mass of water (grams per mole of water).  Don't let that sentence intimidate you.  It's easier than you think.  On the periodic table, find Hydrogen and Oxygen (atomic numbers 1 and 8 and symbols H and O).  (Hydrogen should be the easiest to find as it is the first element on the periodic table.)  Here's a picture of where they are on the periodic table (I'm using a picture from chemistry.about.com periodic tables differ slightly but all have the elemental symbol.)

Now that you have located Hydrogen and Oxygen on the periodic table, look for the atomic mass for each.  Here is a picture showing you where that is at (as previously stated, periodic tables differ slightly.  Some have the atomic mass while others don't and some have the atomic mass above the elemental symbol while others may have it below.  In the one I am using, the atomic mass is below.)

As you can see, the atomic mass for Hydrogen is 1.0079 (we'll round up to 1.008) and Oxygen is 15.9994 (we'll round to 15.999 to give up three places after the decimal).  Now for water, the formula is H2O.  That means there are two Hydrogen atoms and one Oxygen atom.  To find the molecular mass, think of this as a math problem.  Hydrogen is H and Oxygen is O.

(H)*2 + (O)*1 = Molecular Mass

Plug in the atomic masses for the corresponding element.

(1.008)*2 + (15.999)*1 = Molecular Mass

Now solve.

2.016 + 15.999 = 18.015

Yay!  Now we have the molecular mass!  So what to do with this.  Notice that the answer doesn't have any units.  I didn't put them there to make it simpler to calculate the molecular mass without worrying about units just yet.  So were you paying attention?  I actually already said the units for molecular mass.  In case you weren't paying attention, go on ahead and look back.  I'll wait.

Find it?  If you were too lazy to look back, I'll just go on and spoil it for you.  The units for molecular mass are grams per mole.  That means our answer for the molecular mass is actually 18.015 g/mol.  It's now time to put it to some use.

No matter what you're trying to find through stoichiometry, there is a flow that it goes in:

Moles A <=> Grams A <=> Grams B <=> Moles B

Since we are only dealing with water, we only need to worry about Moles A <=> Grams A.  In the problem, we were given the grams of water and we are looking for the moles.  So this is what we are doing:

Grams H2O -> Moles H2O

We know the grams so it now looks like this:

30.2 g H2O -> Moles H2O

Time to use your brain.  What do we know that has moles in it?  Need a hint?  Try the molecular mass!  Remember (grams per MOLE.)  But it also has grams.  That means we need to cancel out grams.  How do we do that?  If a grams is divided by a gram, isn't it one?  (Yes.)  So we have it as moles over grams.

30.2 g H2O -> 1 mol  H2O

               18.015 g

Change the arrow to a multiplication sign.

30.2 g H2O * 1 mol  H2O

             18.015 g

Time to solve!  The grams would cancel out.

30.2 g H2O * 1 mol  H2O

              18.015 g

The final step is to multiply and divide.

30.2 H2O * 1 mol  H2O

         18.015

30.2 mol = 1.676 mol H2O

18.015                            .

So now we know that a cup with 30.2 grams of water has 1.676 moles of water!

Congratulations!  You just completed and solved your first stoichiometry problem!  For right now, I'm just going to do question a.  Check back later for how to solve questions b.  In the mean time, get some practice in!  Solve this stoichiometry question:

There are 13.2 g of NaCl in a bowl.  How many moles are there of NaCl?

You can either comment or email the answer to me and I'll tell you if you're right.  Or if you just want to keep the answer to yourself, I'm going to give the solution when I do the walk through for question b.  (I made up the question myself, so don't think you can cheat and just find it online you sneakies!)

Have a great day!