Stoichiometry, maybe it's the name that scares students. What is stoichiometry? Stoichiometry examines the relationship between substances in a reaction. How can we understand it in real world terms? Let's imagine that we make pasta and have a mean boss. One day, a bag of flour seemed spoiled. Yay! Day off right, NO! Our boss tells us to make the pasta anyway or she'll can us! We only have 2 cups of flour and we need to use every bit... but how many eggs is that? What is the equivalent of eggs in cups? That doesn't make sense. Hmmmmm. One thing we can do to get them in the same terms, is to convert a cup of flour and an egg into weight, into grams... like how much does a cup of flour weigh, how much does an egg weigh... 3.5c Flour + 4 eggs = 2 trays of pasta (120g/1C) (50g/1 egg) Let's make that into an equivalent of our recipe: 420g flour + 200g egg = 2 trays pasta Our recipe is our balanced formula. Stoichiometry step 1 is done. Now, the flour we have to use, our given, is 2C. What does 2c flour weigh * 120g/1C = 240g flour We now have the total weight of the given, or known. Stoichiometry Step 2 is done. Step 3 we are going to take our grams of flour and see how many grams of egg is needed. In order to do this, we need the ratio found in the original recipe. I will set the ratio up so flour is on the bottom and cancels out. We have 240g flour * (200g eggs/420g flour) = 114g egg That's our result for step 3, but that's not entirely helpful! What is 114g of egg in terms of whole eggs? Step 4: Multiply 114g * by the rate with grams on the bottom (1egg/50g) = 2.28 or about 2 and a quarter eggs. Our problem is finished. We have examined the relationship between substances in a reaction, just like stoichiometry. In Chemical terms, the reactants are just ingredients you aren’t familiar with and the products doesn't make as much sense to you now, as pasta does. Stoichiometry: A Real-World Overview © 2009 Jessica DiBacco