Question

4al(s) + 3o₂(g) → 2al₂o₃(s)
the reaction began with 32.0 g o₂.
the reaction used 12.0 g o₂.
what mass of excess reactant (oxygen) remains after the reaction is complete?

Explanation:

Step1: Identify given masses

Initial mass of $\ce{O2}$: $32.0\ \text{g}$, Used mass of $\ce{O2}$: $12.0\ \text{g}$

Step2: Calculate remaining mass

Subtract used mass from initial mass: $32.0 - 12.0 = 20.0$

Answer:

$20.0\ \text{g}$