Limiting Reagent:
The limiting reagent is the reactant that is completely consumed in a chemical reaction, thereby limiting the amount of product that can be formed. It determines the theoretical yield of the reaction. The reactant that is not completely consumed is called the excess reagent.

Finding the Limiting Reagent:
To determine the limiting reagent in a chemical reaction, you need to compare the stoichiometric ratios of the reactants to the desired product. The reactant that produces the smallest amount of product is the limiting reagent.

Here's a step-by-step example to find the limiting reagent:

Example:
Consider the reaction between hydrogen gas (H2) and oxygen gas (O2) to form water (H2O):

2H2 + O2 → 2H2O

Let's say we have 4 moles of H2 and 3 moles of O2. We need to determine which is the limiting reagent.

1. Calculate the moles of product:
- For H2: 4 moles H2 × (2 moles H2O / 2 moles H2) = 4 moles H2O
- For O2: 3 moles O2 × (2 moles H2O / 1 mole O2) = 6 moles H2O

2. Compare the moles of product:
- H2 produces 4 moles of H2O.
- O2 produces 6 moles of H2O.

3. Identify the limiting reagent:
- Since H2 produces a smaller amount of product (4 moles), it is the limiting reagent.

4. Calculate the excess reagent:
- Excess O2 = moles of O2 reacted - moles of O2 required by limiting reagent
- Excess O2 = 3 moles O2 - (4 moles H2 × 1 mole O2 / 2 moles H2) = 1 mole O2

Conclusion:
In the given example, hydrogen gas (H2) is the limiting reagent because it produces a smaller amount of product. Oxygen gas (O2) is the excess reagent, with 1 mole remaining after the reaction.

By identifying the limiting reagent, we can determine the maximum amount of product that can be formed in a reaction. This information is crucial for calculating the theoretical yield and understanding the efficiency of a chemical process.