Analía Bellizzi – Chemistry Classes

Ronald Reagan High School

Stoichiometry Notes

Stoichiometry

Definition:

The term stoichiometry is derived from two Greek words: stoicheion (meaning “element”) and metron (meaning “measure”).

Stoichiometry refers to the quantitative relationship among reactants and products in a chemical reaction that is represented through a chemical equation. 

Stoichiometry calculations are based on the Law of conservation of mass. Since atoms cannot be destroyed or created -through chemical reactions-, the number and kind of atoms before and after the reactions are always the same. 

Steps to resolve an stoichiometry question:

    1. Construct the chemical equation based on the chemical reaction you want to do your calculations.

      Example: Combustion of Pentane

          C5H12 + O2 —> CO2 + H2O

       

    2. Balance the equation. (If you do not balance your equation, all calculations may be wrong!)

      C5H12 + 8 O2 —> 5 CO2 + 6 H2

       

    3. Be sure you understand the number of moles reacting are represented by the coefficients in the reaction.

       mole of C5H12 + 8 moles of O2 —>  5 CO2 + 6 moles of H2O

       

    4. Verify that the masses of the reactants and products are the same. (Law of conservation of mass)

      72 g of C5H12 + 256 g of O2 —>   220 g of CO2 + 108 g of H2

       

    5. Then, and ONLY THEN, you can begin with your calculations

You have to follow the steps below to get any of the answers.  

ANOTHER METHOD TO RESOLVE THE PROBLEMS

For calculations, refer to the exercises in the videos below

 

 Theorethical yield, actual yield and % Yield.

Theoretical yield

The amount of product that could possibly be produced in a given reaction, calculated according to the starting amount of the limiting reagent.

Actual yield

The amount of product actually obtained in a chemical reaction.

Percent yield

Refers to the efficiency of a chemical reaction; defined as the   

% Yield =      actual yield      x 100     
                           theoretical yield 

This is an external video (not mine but very good!)