Question

How to find the limiting reactant given moles?

Answer 1

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.