Question

What are common mistakes students make with limiting reagent problems?

Answer 1

Two common errors come to mind for me. Not converting amounts of reactants to moles and not using the coefficients from a balanced equation.

Let's consider the following reaction
`H_2` + `N_2` -> `NH_3`

We need to balance the equation to do any stoich work
3`H_2` + `N_2` -> 2`NH_3`

If we have 6g of `H_2` available for the reaction and 14g of `N_2` available many students will say that the hydrogen is the limiting reactant because there are fewer grams of hydrogen. In this case the nitrogen is the limiting reactant, let's consider why...

6g of hydrogen is equal to 3 moles (the number from the balanced equation) of `H_2`

14g of Nitrogen is only a half mole of diatomic nitrogen (`N_2`)

Using the coefficients from the balanced equation, we can conclude that if we only have 0.5mole `N_2`, only 1.5 mole `H_2` will be able to react. This allows us to conclude that `H_2` is the excess reactant since there is more hydrogen present than will be needed to use up all the nitrogen.

Here is a video to help further with this concept.