Identifying limiting reactants can be tricky for students, and one big reason is misunderstanding the ratios in a chemical equation.

When you balance a chemical equation, the numbers in front of each substance tell you how much is needed. A common mistake is not paying attention to these numbers. For example, in the equation:

$aA + bB \rightarrow cC$

Many students only look at the amounts of A and B without considering the numbers $a$ and $b$.

Another mistake is not changing units correctly. Sometimes students think they can just compare weights or volumes without changing them to moles first. You cannot say, "I have 5 grams of A and 10 grams of B," and find the limiting reactant right away. You need to convert both amounts to moles using their molar masses. Then, you can use the ratios from the balanced equation.

Also, a lot of students forget to think about leftover reactants. After finding the limiting reactant, they might not realize that some of the other reactants will still be left over. It’s important to track how much of each reactant is used and what is left.

Lastly, many students forget to check their calculations. If you make a mistake in converting moles or in basic math, you might end up with the wrong answer about which reactant is limiting. Always go back and look at each step to make sure your work is correct.

In short, to avoid these mistakes, remember to:

1. Use the ratios from the balanced equation correctly.
2. Always convert to moles.
3. Keep track of leftover reactants.
4. Double-check your math.

By staying aware of these common errors, you can confidently find the limiting and excess reactants in any reaction!