Stoichiometry

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Recipe Card

We have already learned that a recipe can be modified to produce a desired quantity of something. For example if a recipe to produce a cake called for 2 cups of flour, 1 cup of sugar, and 1 egg, how many eggs would you need to make 2 cakes? If you wanted to make 2 cakes you would need to double the recipe and so you would need 2 eggs instead of 1 egg.

A balanced chemical equation works just like a recipe. We can modify the chemical equation to produce a desired outcome as long as we do the same modification to everything in the chemical equation.

Lets try an example of a calculation that uses a balanced chemical equation. The calculation we are going to perform is apart of a branch of chemistry that is called stoichiometry. Stoichiometry deals with the quantities in chemical reactions.

Remember these steps to solve any stoichiometry problem:

  1. Write the “given” with units
  2. Convert the “given” into moles
  3. Multiply by the mole ratio that uses the coefficients from the balanced chemical equation to compare the given to the new quantity
  4. Convert your answer into the desired units (grams, litres of gas, etc.)

 

 

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Example 1: Using the mole only

Hydrogen and oxygen gas will undergo a highly exothermic reaction to produce water according the following unbalanced chemical equation,

chemical equation

If 3.7 moles of hydrogen gas reacts with an abundance of oxygen gas how many moles of water with be produced?

 

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Example 2: Using grams

Natural gas is used to heat many homes in north america and it is mainly comprised of methane gas. Methane gas undergoes a combustion reaction according to the following balanced chemical equation,

Methane Reaction

 If 17.3 grams of carbon dioxide is produced from the combustion of methane, how many grams of methane was used in the reaction?

 

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Example 3: Using litres of gas

Propane is commonly used as the fuel in a barbecue. The propane is a liquid inside of the tank due to the high pressure inside the tank, however, it becomes a gas as it exits the tank. Propane gas burns in oxygen according the the following chemical equation,

Propane Reaction

If your barbecue burned 12.54 litres of propane gas, how many litres of oxygen would be required to complete the combustion?

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