A Limiting Reagent is the reagent that limits the amount of products that can be formed.
For example, nitrogen gas is prepared by passing ammonia gas over solid copper(II) oxide at high temperatures. The other products are solid copper and water vapor.
2 NH3(g) + 3 CuO(s) → N2(g) + 3 Cu(s) + 3 H2O(g)
If 18.1 g of NH3 are reacted with 90.4 g of CuO, which is the limiting reagent? How many grams of N2 will be formed?
First we compute the number of moles of NH3 (M.W. = 17.031 g/mole) and the number of moles of CuO (M.W. = 79.5 g/mole).
To determine which reagent is limiting we use the mole ratio from the chemical equation to convert moles NH3 to moles CuO.
So, only 1.14 moles of CuO is available, therefore CuO is the limiting reagent. That is, CuO will run out before the NH3 does.
The mass of N2 produced will be