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Stoichometry of chemical equations

My chemistry book asks the question:

Write the balanced equation for the complete combustion of each of the following compounds:

a. ethane, C2 H6

The book has the answer which reads:

2C2h6 + 7O2 I don't understand where the formula for 7 02

b. cyclopropane

I understand the formula C3H10 but I don't understand the "addition" of a 2 in front - and where the number for the oxygen comes in.
Answer says: 2C3H6 + 9 02

The book only gives the answer but does not explain specifically the math.

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Solution Preview

To balance out an equation we have to think about the fact that during a chemical reaction mass ( i.e. number of atoms) cannot be changed. That means we cannot create or destroy any atom during a chemical reaction.

So, we have to have the same number of every sort of atoms on side of the reactants and products.

To solve the question we write down first the complete chemical equation for combustion. If we have alkanes the products are ...

Solution Summary

The solution describes step-by-step how to formulate a stoichiometric correct chemical equation for combustion of two alkanes.