Wednesday, January 06, 2010


Go on. Say it.

You know you want to.

Stoichiometry, if you have forgotten, is the "math" part of chemistry. Balancing equations is the first thing you learn. Later, finding out how much of this-n-that you need to produce so much product. This is why you can't take Chemistry until you have had algebra.

Last night, Blackeyed Susan asked for help with her homework. At first I stared at the page of problems. I referred to her textbook. (have I told you before how inferior her public school text is compared to my beloved Apologia science books?) The practice problems were so easy you could actually do them in your head, so those were no biggie. We worked on the next problems for awhile, and then the lightbulb went on for me. The answer was in the Unit Multipliers.

You remember unit multipliers, don't you? You actually use them every day. Say you want to find out how many feet are in three miles. You multiply 3 feet by 5280 feet Per mile (imagine that as a fraction, I don't know how to type math symbols...) and get 15840 feet.

Here is one simple problem: How many moles of ammonia are produced when .60 mol of nitrogen reacts with hydrogen?

Here's the equation: N2 + 3H2 = 2NH3 (except that in chemistry the = sign is really an arrow)

This means that you need 1 mole of nitrogen combined with 3 moles hydrogen to make 2 moles of ammonia. All you need to know is the ratio. If you need 1 mol of N2 to make 2 mol of NH3, then to find your answer you multiply .6o mol of N2 by the unit multiplier 2 mol NH3 *over* 1 mol N2. Cancel, multiply, and your answer is 1.2 mol NH3.

We also did mass. Calculate the number of grams of NH3 produced by the reaction of 5.40 g of H2 with an excess of N2. (equation: N2 + 3H2 =2NH3)

This required three unit multipliers to get from grams of H2 to grams of NH3. Or how about this one? How many molecules of oxygen are produced when 29.2 grams of water is decomposed by electrolysis according to this equation?

2H20 (arrow with the word electricity over it) = 2H2 + O2

Hello. How many molecules? Yep. Easy-schmeasy with Avogadro's number, the number of molecules in a mole of anything: 6.02 x 10-to-the-23rd. It becomes your final unit multiplier.


Oh. You want the answer, don't you? Here's your math:

29.2 g H20 x 1 mol H2O (0ver)18 g H2O (you get the mass of one mole of something from the periodic table) x 1 mol O2 (over)2 mol H2O (your ratio) x 6.02 x 10 to the 23rd molecules O2 (over)1 mol O2. Cancel, cancel, cancel, multiply. Your answer (and I know you'll do the math because you can't resist):

4.88 x 10-to-the-23rd molecules of O2.

We also did gases (mass is 22.4L per mole) so it got pretty complicated there. But it got so exciting for me that I wanted to just take the book from Susan and just do the work myself!

Gee, I miss homeschooling.

(and did I tell you that it has been 36 years since I have had any chemistry?)

Have a fun-filled, math-filled day!

by the way, if you care, I blogged on my other blog today as well!


Inglesidemom said...

I did not understand one word in that post. I can barely multiply without using my fingers.

I can, however, carry a tune and bake a great loaf of bread. Of course, I think you can do that too!

G.L.H. said...

You are so cute!