Final answer:
The amount of excess reactant left after a reaction has gone to completion, we need to identify the limiting reactant. In this case, substance B is the limiting reactant, while substance A is in excess. Since 3.0 moles of substance A reacts with 6.0 moles of substance B.
Step-by-step explanation:
In order to determine the amount of excess reactant left after a reaction has gone to completion, we need to identify the limiting reactant first. The limiting reactant is the reactant that is completely consumed in the reaction, thus determining the maximum amount of product that can be formed. In this case, we can compare the number of moles of substance A and substance B and see which one is present in excess.
Since 3.0 moles of substance A reacts with 6.0 moles of substance B, we can see that substance B is the limiting reactant because it is in stoichiometric excess. Therefore, after the reaction has gone to completion, there will be no moles of substance B left over. However, there will be some moles of substance A remaining as it is in excess.