Explain what a limiting reactant is and describe the process to determine the limiting reactant in the example below:

Question

Given 3.4 grams of x element with a molar mass of 24 g/mol and 4.2 grams of y element with a molar mass of 35 g/mol. How much of compound xy2 can be generated?

   x + 2y -> xy2?

Answer 1

A limiting reactant is a reactant in a chemical equation that runs out before any other reactant. It therefore limits the amount of product that can be formed.

There are a couple of ways of determining which reactant is limiting. One relatively simple way is to divide the moles of each reactant by the corresponding coefficient in the balanced equation. Whichever value comes out less represents the limiting reactant. We will use this method in the current problem.

x + 2y ==> xy₂ .. balanced equation

moles of x present = 3.4 g x 1 mol x / 24 g = 0.142 mols x (÷1->0.142)

moles of y present = 4.2 g x 1 mol y / 35 g = 0.12 mols y (÷2->0.06)

Since 0.06 is less than 0.142, this tells us that y is the limiting reactant (it will be used up before x is)

How much xy₂ can be formed? We use the moles of limiting reactant to predict how much product can be formed.

0.12 mols y x 1 mol xy₂ / 2 mols y = 0.060 mols xy₂ can be formed

If you want to know the grams of xy₂ formed, we would need to know the molar mass of xy₂. This can be calculated as follows: 24 g + 35 g + 35 g = 94 g/mol

0.060 mols xy₂ x 94 g / mol = 5.6 g xy₂ can be formed