How to find the limiting reactant given moles?

1 Answer
Nathan L.
Jul 30, 2017

If you're given the moles present of each reactant, and asked to find the limiting reactant of a certain reaction, then the simplest way to find which is limiting is to divide each value by that substance's respective coefficient in the (balanced) chemical equation; whichever value is smallest is the limiting reactant.

As an example, let's say we have the reaction

#2"H"_2(g) + "O"_2(g) rarr 2"H"_2"O"(g)#

and that we're given

  • #3# moles of hydrogen

  • #2# moles of oxygen

The limiting reactant is that whose value is smallest after dividing the mole number by their coefficient:

#"H"_2#: #(3color(white)(l)"mol")/(2color(white)(l)"(coefficient)") = ul(1.5)#

#"O"_2#: #(2color(white)(l)"mol")/(1color(white)(l)"(coefficient)") = 2#

We see that the number for hydrogen is the lower value, so hydrogen is the limiting reagent.

Notice also how the limiting reactant isn't necessarily the reactant present in a larger quantity; there were #3# #"mol H"_2# and #2# #"mol O"_2#, but hydrogen was limiting.

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