Stoichiometry Kelley Kuhn CCA

What the heck is stoichiometry? Stoichiometry is what we use in chemistry to solve problems. For example, you can use stoichiometry to calculate how much of a product you will make from a given amount of a reactant. You must have a BALANCED chemical equation for stoichiometry to work.

Stoichiometry: It’s a process… 1. Make sure your chemical equation is balanced. 2. Circle the substances that are being considered in the problem. 3. Begin with the quantity in grams provided in the problem. 4. Convert this quantity from grams to moles. 5. Set up the mole ratio with the starting substance on bottom and the desired substance on top. 6. Convert this substance from moles back to grams.

Example: C 3 H O 2 ---> 3 CO H 2 O How many grams of oxygen are required to completely react 28.5 g of propane (C3H8)?

Stoichiometry, redux For any chemical reaction, one reactant may be present in excess and the other may be present in a limiting quantity. Excess reagent/reactant: the reactant which is present in excess and does NOT limit how much product will be made. Limiting reagent/reactant: the reactant which limits how much product is made because it is present in a proportionally less amount. In a limiting reagent problem, you must set up TWO stoichiometric equations, one for each reactant. Whichever reactant generates LESS product is considered the limiting reagent and this amount of product is the amount actually made (aka theoretical yield).

Some associated terminology: Theoretical yield: the yield anticipated based on a stoichiometric calculation. Actual yield: the yield actually generated in lab. Percent yield = actual yield x 100 theoretical yield Percent error: theoretical yield - actual yield x 100 theoretical yield Actual yield = (theo. Yield)(percent yield) 100