To find out which reactant is limiting when calculating yields, you can follow these simple steps:

1. Write the Balanced Chemical Equation:
   Start by making sure your chemical equation is balanced. For example, consider this reaction:
   $aA + bB \rightarrow cC$
   Here, $A$, $B$, and $C$ are the substances involved, and $a$, $b$, and $c$ are numbers that tell us how many of each are needed.

2. Convert All Given Reactants to Moles:
   Change the amount of each reactant from grams or liters to moles using their molar masses. For example, if you have 10 grams of substance $A$ and its molar mass is 20 g/mol:
   $\text{Moles of } A = \frac{10 \, \text{g}}{20 \, \text{g/mol}} = 0.5 \, \text{mol}$

3. Calculate the Moles of Reactants Needed:
   Look at the numbers from your balanced equation to see how many moles of each reactant you need. If the equation says you need $1A + 2B$, then to use $0.5$ moles of $A$, you will need $1.0$ mole of $B$.

4. Compare Available Moles to Required Moles:
   Check which reactant will run out first. If you only have $0.5$ moles of $B$, then $B$ is the limiting reactant because you need $1.0$ mole, while $A$ is in excess.

5. Calculate Theoretical Yield:
   Find out how much product you can theoretically create based on the limiting reactant. If $C$ is made from the limiting reactant $B$, and the ratio shows that $2B$ makes $1C$, then:
   $\text{Moles of } C = \frac{0.5 \, \text{mol B}}{2} = 0.25 \, \text{mol C}$
   Change this into grams using the molar mass of $C$ for the theoretical yield.

6. Calculate Percent Yield:
   Finally, compare the actual yield from your experiment to the theoretical yield:
   $\text{Percent Yield} = \left( \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \right) \times 100\%$

By following these steps, you can easily find out which reactant is limiting and do yield calculations correctly.