Limiting Reagent Made Easy: Stoichiometry Tutorial Part 5

This video, the final in a stoichiometry series, will teach how to identify limiting or excess reactants and calculate reaction yields. The limiting reagent is the reactant that limits the amount of product formed.

Using a s'mores recipe, the video demonstrates how to determine the limiting ingredient. With 27 graham crackers, 40 marshmallows, and 12 chocolate bars, and a modified recipe needing 4 marshmallows, 2 graham crackers, and 1 chocolate bar for two s'mores, marshmallows are identified as the limiting reagent because they can only make 20 s'mores, the least amount.

The theoretical yield is the amount of product made by the limiting reagent if everything works perfectly. In the s'mores example, the theoretical yield is 20 s'mores, as marshmallows ran out at that point.

The video applies the concept to a chemical reaction: combining 75g of magnesium with 45g of nitrogen to form magnesium nitride. First, balance the reaction: 3Mg + N2 → Mg3N2. Then, calculate the molar masses for each reactant and product.

To find the theoretical yield, calculate the amount of magnesium nitride producable from each reactant. From 75g of magnesium, 104g of magnesium nitride can be made. From 45g of nitrogen, 162g of magnesium nitride can be made. Therefore, 104g of magnesium nitride is the theoretical yield, and magnesium is the limiting reagent.

If 88.7g of magnesium nitride were actually collected in the lab, the percent yield is calculated as (actual yield / theoretical yield) * 100. So, (88.7g / 104g) * 100% = 85% percent yield.