A chemist exploring potential fuels for use in outer space is considering using vaporized aluminum chloride as a fuel. What is the coefficient of O2 (g) when the following chemical equation is balanced using the smallest whole number integer coefficients?

9

Explanation:

The reaction given is:

AlCl₃ (g) + O₂ (g) → Al₂O₃ (s) + ClO (g)

To be balanced, all the elements must have the same quantity on both sides of the chemical equation. Let's start by Al, which has 2 in the right and 1 on the left. We must multiply AlCl₃ by 2:

2AlCl₃ (g) + O₂ (g) → Al₂O₃ (s) + ClO (g)

Now, let's do the same thing to Cl, which has 6 in the left and 1 in the right, so we multiply ClO by 6:

2AlCl₃ (g) + O₂ (g) → Al₂O₃ (s) + 6ClO (g)

And now for O, which has 2 in the left and 9 in the right, so we multiply O₂ by 9/2:

2AlCl₃ (g) + (9/2) O₂ (g) → Al₂O₃ (s) + 6ClO (g)

Because the question wants whole numbers, let's multiply all the coefficients by 2:

4AlCl₃ (g) + 9O₂ (g) → 2Al₂O₃ (s) + 12ClO (g)

So, the coefficient of O₂ is 9.