If 4.0 moles of x and 4.0 moles of y react according to the hypothetical reaction below, how many moles of the excess reactant will be left over at the end of the reaction? 2x + 3y yields x2y3

1.3 mol y left over
2.7 mol y left over
1.3 mol x left over
2.7 mol x left over

2X + 3Y --> X2Y3

You have initially 4 moles of both X and Y. 
2 moles of X and 3 moles of Y are needed to make X2Y3.

4 moles X * (3 moles Y/2 moles X) = 6 moles Y
We do not have 6 moles of y, which means that Y is the limiting reactant.

4 moles Y * (2 moles X/3 moles Y) = 2.667 moles X

We start off with 4 moles of X, and we react 2.667 moles of X.
We start off with 4 moles of Y, and we react all 4 moles of Y. (Remember, this is the limiting reactant.)

The reactant that will be in excess is X, and it will have 4 - 2.667 = 1.333 moles of X remaining.

Your final correct answer is 1.3 mol X left over.

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