To find extra reactants in stoichiometric calculations, just follow these simple steps:

1. Start with the Balanced Equation: Make sure that the chemical reaction is balanced. This means the number of atoms for each element is the same on both sides. This helps you get the correct ratios.

2. Find the Moles of Reactants: Change the weights of the reactants into moles. You can use this formula:
   [ \text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}} ]

3. Use Stoichiometric Ratios: Look at the mole ratio of the reactants and compare it to the ratio from the balanced equation.

4. Calculate Needed Moles: For each reactant, find out how many moles are needed to react completely with the one that runs out first (the limiting reactant). Use the ratios from the balanced equation.

   • For example, if you have a reaction like (aA + bB \rightarrow cC) and (A) is the limiting reactant, then you calculate:
     [ \text{Needed moles of } B = \frac{b}{a} \times \text{moles of } A ]

5. Find Excess Reactant: The reactant that is left over after the limiting reactant is all used up is the excess reactant.

By following these steps, you can easily figure out which reactants are not fully used in a chemical reaction.