Welcome to limiting reagents! In any chemical reaction, we often have multiple reactants that combine to form products. The limiting reagent is the reactant that runs out first, determining how much product we can make. Think of it like baking cookies - if you have enough flour for 100 cookies but only enough chocolate chips for 50, the chocolate chips limit how many cookies you can actually bake.
The first step in determining limiting reagents is to write and balance the chemical equation. This gives us the stoichiometric ratios we need for our calculations. For example, in the formation of water, two molecules of hydrogen gas react with one molecule of oxygen gas to produce two molecules of water. The balanced equation shows us that hydrogen and oxygen react in a 2 to 1 mole ratio.
The second step is to convert the given amounts of each reactant to moles using their molar masses. For example, if we have 4.0 grams of hydrogen gas and 32.0 grams of oxygen gas, we divide each mass by its respective molar mass. Hydrogen has a molar mass of 2.0 grams per mole, giving us 2.0 moles. Oxygen has a molar mass of 32.0 grams per mole, giving us 1.0 mole. Now we can compare these amounts using the balanced equation.
Now we compare the actual amounts with the stoichiometric requirements. Our balanced equation shows that hydrogen and oxygen react in a 2 to 1 mole ratio. We have exactly 2.0 moles of hydrogen and 1.0 mole of oxygen, which matches the required ratio perfectly. In this case, neither reactant is limiting - they are in perfect stoichiometric proportions and will both be completely consumed to produce 2.0 moles of water.
Let's work through an example where we do have a limiting reagent. Given 3.0 grams of hydrogen and 16.0 grams of oxygen, we first convert to moles: 1.5 moles of hydrogen and 0.5 moles of oxygen. According to our balanced equation, 1.5 moles of hydrogen would need 0.75 moles of oxygen to react completely. But we only have 0.5 moles of oxygen available. Therefore, oxygen is the limiting reagent, and we can only produce 1.0 mole of water, with some hydrogen left over as excess.