You need 4.04 grams of hydrogen gas. Beyond Perfect Recipes: Limiting and Excess Reactants In a real chemistry lab, you rarely have the exact perfect amounts of both reactants. Usually, you have more of one and less of another. This introduces the concept of the limiting reactant (or limiting reagent).
Imagine you are baking a cake. You know that to make one cake, you need 2 cups of flour, 1 cup of sugar, and 3 eggs. If you want to make three cakes, you simply multiply every ingredient by three. Chemistry works in a very similar way, but instead of cakes, we are making molecules. This mathematical “recipe book” of chemistry is called stoichiometry (pronounced stoy-kee-ah-muh-tree ).
Chemists use the following formula to measure their efficiency: stoikiometri
Using the periodic table, we can convert between grams (what you can weigh on a scale) and moles (the number of particles). This is the first step in most stoichiometry problems. Let’s walk through a classic problem. Suppose you have 36 grams of water (H₂O). How many grams of hydrogen gas (H₂) are needed to make that water, assuming you have unlimited oxygen?
In chemistry, you must always identify the limiting reactant before you can calculate how much product you will actually get. Even when you do the math perfectly, real experiments rarely produce the theoretical amount of product. Some product may stick to the glassware, evaporate, or react in a side reaction. The amount you calculate is the theoretical yield (the perfect result). The amount you actually measure in the lab is the actual yield . You need 4
Look at the coefficients: For every 2 moles of H₂O produced, you need 2 moles of H₂. The ratio of H₂ to H₂O is 2:2, which simplifies to 1:1. Moles of H₂ needed = 2.00 moles H₂O × (2 mol H₂ / 2 mol H₂O) = 2.00 moles H₂.
The molar mass of H₂O = (2 × 1.01) + 16.00 = 18.02 g/mol. Moles of H₂O = (36 g) / (18.02 g/mol) ≈ 2.00 moles. This introduces the concept of the limiting reactant
The molar mass of H₂ = 2 × 1.01 = 2.02 g/mol. Grams of H₂ = 2.00 moles × 2.02 g/mol = 4.04 grams.
The other reactants are called excess reactants .
One mole is an enormous number: 6.022 x 10²³ particles (Avogadro's number). Think of the mole as the chemist’s “dozen.” Just as a dozen always means 12 items, a mole always means 6.022 x 10²³ items.