Question

What coefficient is placed in front of `O_2` to complete the balancing of the following equation `C_5H_8 + ?O_2 -> 5CO_2 + 4H_2O`?

Answer 1

`C_5H_8+7O_2->5CO_2+4H_2O`
The coefficient for `O_2` is 7.

Explanation:

You're off to a nice start.

The balanced form of the chemical equation you have provided is:
`C_5H_8+7O_2->5CO_2+4H_2O`