Question

My teacher says the correct answer is 7, but I don't understand why. Can anyone explain?

Answer 1

`C_5H_8(l) +7O_2(g) rarr 5CO_2(g) + 4H_2O(g)`

Explanation:

Chemical reactions conserve mass and charge. What does this mean? It means that if there are `292*g` of reactants, there can be AT MOST `292*g` of products, and the stoichiometrically balanced equation reflects this. Likewise, the left hand side of the equation was neutral; the right hand side is also necessarily neutral.

And are there `292*g` of reactants? Let's count them. `(5xx12.011+8xx1.008+7xx32.00)*g*mol^-1=292*g*mol^-1" of reaction as written."`

If you don't see this immediately, voice your objection, and someone here will make another attempt. The idea of stoichiometry, of mass equivalence, is fundamental to the study of chemistry. (It is fundamental to accounting also! Why?) As a start count up how many carbon, hydrogen, and oxygen atoms on the left hand side of the equation, and compare them to the number of `C,H,O` atoms on the right hand side.

Here is a similar question on redox equations. Once again, it is required to balance both mass and charge on each side of the equation. See here for a similar answer on stoichiometry.