If 3.0 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? x + 2y xy2 2.0 mol x left over 1.0 mol x left over 2.0 mol y left over 1.0 mol y left over

Answer is: 1.0 mol X left over. 
Chemical reaction: X + 2Y → XY₂.
n(X) = 3,0 mol, excess reactant.
n(Y) = 4,0 mol, limiting reagent.
n - amount of substance.
from reaction: n(X) : n(Y) = 1 : 2.
n(X) : 4 mol = 1 : 2.
n(X) = 2 mol, that reacts.
excess of X: 3 mol - 2 mol = 1 mol.

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