L11: Chemical Quantities

Balanced equations tell us the relative numbers of molecules of reactants and products. In the equation C + O2 → CO2, we see that one atom of carbon combines with one molecule of O2 to make one molecule of CO2 which is carbon dioxide. If you start the reaction with exactly one atom of carbon, you can tell from the equation that you would need exactly one molecule of oxygen gas to fully react with all the carbon and produce one molecule of carbon dioxide. Conversely, if you want to produce exactly two molecules of carbon dioxide, you would start with two atoms of carbon and two molecules of oxygen gas. Knowing the exact ratios between the reactants and products in a reaction allows chemical manufacturers to make products in a cost-effective manner. They can combine the right amounts of reactants to make the amount of product that they need.

A similar situation occurs in a tricycle manufacturing plant. Three wheels, one seat, and one set of handlebars combine to make one tricycle. This could be written as 3W + 1S + 1Hb → 1T. The most cost effective way to make one tricycle would be to buy three wheels, one seat, and one set of handlebars. You could also make one tricycle by buying six wheels, two seats and one set of handlebars, but after assembling the tricycle, you would have three wheels and one seat left over. In this case, the number of handlebars limits how many tricycles you can make. In Lesson 11, we will see how the quantitative relationship between reactants and products in chemical reactions can be used to predict how much of a reactant is needed or how much product can be formed.

Objectives for Lesson 11

After completing this lesson, you will be able to: 

  • Write the number of moles and the mass of both the products and the reactants in a chemical reaction from information given in a balanced equation 
  • To learn to use a balanced equation to determine relationships between moles of reactants and moles of products
  • Calculate the mass of the products produced based on the mass of the reactants in a chemical reaction 
  • Identify the limiting reactant in a reaction 
  • Apply the limiting reactant to do stoichiometric calculations 
  • Calculate percentage yield from the ratio of actual yield to theoretical yield