Answer:
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.