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Calculations Involving Chemical Reactions 

In this section, we will look at some chemical equations and find what we can do with the information represented in a chemical equation. The equation shown below represents the reaction of methane and oxygen to form carbon dioxide and water.


From this balanced equation we can infer many things. Let's consider a few.
  1. One molecule of methane reacts with 2 molecules of oxygen to form 1 molecule of carbon dioxide and 2 molecules of water.
  2. We can also say that 1 mole of methane reacts with 2 moles of oxygen to form 1 mole of carbon dioxide and 2 moles of water.
  3. Since one mole contains Avogadro number of molecules, we can say that 6.023 x 1023 molecules of methane reacts with 1.2046 x 1024 (= 2 x 6.023 x 1023) molecules of oxygen to form 6.023 x 1023 molecules of carbon dioxide and 1.2046 x 1024 molecules of water.
  4. We can confidently say that 16 g of methane reacts with 64 g of oxygen to form 44 g of carbon dioxide and 36 g of water.

Example 2-4
Calculate the number of moles of water produced when 5.25 moles of methane undergo the reaction depicted below. (Assume there is plenty of oxygen for the reaction)




From the equation, it is clear that for every mole of methane, 2 moles of water are formed. So without any elaborate calculations, you should be able to come up with the correct answer. It is very much like a ratio problem. Since there are 5.25 moles of methane, 10.5 moles of water will be formed.

Number of moles of water formed = 5.25 x 2 = 10.5 moles


Example 2-5
110 g of CO2 were formed as a result of the reaction shown below. How many grams of oxygen must have reacted to form that much  carbon dioxide?




According to the equation, 2 moles of oxygen result in 1 mole of carbon dioxide. For example, if 2 moles of carbon dioxide were formed, 4 moles of oxygen must have reacted. Here, the amount of carbon dioxide formed is given in terms of grams. So the first step is to convert the grams to moles.



Hence, 5 moles of oxygen must have reacted to form 2.5 moles of carbon dioxide. But the question asks for this quantity in grams. So the final step is to convert moles to grams.



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