If 20 atoms of aluminum react with 45 molecules of chlorine gas, which reactant is limiting and how many more atoms/molecules would be required to use up all the reactants?

Explanation:

According to reaction, 2 atoms of aluminum gas reacts with 3 molecules of chlorine gas.

Then 20 atoms of aluminum will react with :

of chlorine

Aluminum is a limiting reagent.

Molecules of chlorine left unreacted: 45 - 30  molecules = 15 molecules

Then 15 molecules of chlorine gas will react with :

of aluminum

10 more atoms of aluminum will required to use up all chlorine.