How to find the limiting reactant given moles?

1 Answer
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.