Another term for limiting reagent.

To determine what is the limiting reactant in a chemical reaction one must (of course) know how much of each reactant is present and then convert those values from grams to moles. Then arbitrarily select a reactant and figure how many moles of the other reactants are required to completely consume the first reactant. If the amount required exceeds the number of moles present, then you have found the limiting reactant.

For example, suppose we have 50g of Mg(OH)2 and HCl in the following reaction:

Mg(OH)2 + 2 HCl --> MgCl2 + 2 H2O

First we need to figure out how many moles of each reactant we have.

Mg(OH)2 has a molar mass of 58.3 g/mol so 50g Mg(OH)2 would be 0.858 mol Mg(OH)2.

HCl has a molar mass of 36.5 g/mol so 50g HCl would be 1.37 mol HCl.

So now we arbitrarily pick one of the reactants and measure how much of the other would be required to consume the first completely. Let's try Mg(OH)2.

Since 2 moles of HCl are required to react 1 mole of Mg(OH)2 we multiply 0.858mol by 2 and get 1.72mol. Clearly this makes HCl the limiting reactant because we do not have enough of it to fully react the Mg(OH)2

If we had decided to look at HCl instead of Mg(OH)2 we would've again seen that Mg(OH)2 could not be a limiting reactant in this reaction. Again since 2 moles of HCl are required to react 1 mole of Mg(OH)2 we must multiply 1.37mol by 1/2 which gives us 0.685 mol. Since this is less than the number of moles of Mg(OH)2 we know it is not the limiting reactant.