Final answer:
To find the moles of excess reactant left, calculate the moles of B used by multiplying the moles of A by the mole ratio and subtract this from the initial moles of B.
Step-by-step explanation:
To determine the moles of excess reactant left over, we need to find the limiting reactant first. The balanced equation tells us that 1 mole of A reacts with 3 moles of B to form 4 moles of C. So, the mole ratio of A to B is 1:3. Since we have 3.0 moles of A and 6 moles of B, we can determine the limiting reactant by comparing the moles of A to B. Since we have more moles of B than A, A is the limiting reactant.
To find the moles of excess reactant left, we can use the stoichiometry of the reaction. Since 1 mole of A reacts with 3 moles of B, we can multiply the moles of A by the mole ratio to find the moles of B used:
Moles of B used = 3.0 mol A x (3 mol B / 1 mol A) = 9.0 mol B
Since we started with 6 moles of B, we can subtract the moles of B used from the initial moles to find the excess:
Moles of excess B = 6 moles B - 9.0 moles B = -3.0 moles B