If 2.0 moles of a and 5.0 moles of b react according to the hypothetical reaction below, how many moles of the excess reactant will be left over at the end of the reaction?

a + 2b → ab2 (d, right? )

0.50 mol a left over

0.50 mol b left over

1.0 mol a left over

1.0 mol b left over

To determine the amount of excess reactant left, we need to first identify which is the limiting and the excess reactant. We do as follows:

A + 2B → AB2

2 mol A ( 2 mol B / 1 mol A) = 4 mol B needed to be consumed completely
5 mol B ( 1 mol A / 2 mol B) = 2.5 mol A needed to be consumed completely

Therefore, the limiting would be reactant A and the excess would be reactant B. The amount of the excess would be:

5 mol B - 4 mol B = 1 mol B <LAST OPTION

Yes it would be a physical change.