To identify the limiting and excess reagents in a chemical reaction, you need to know the balanced equation and the amounts (in moles or grams) of each reactant you have.

Let's take the reaction between hydrogen (H2) and oxygen (O2) to form water (H2O) as an example:

2H2 + O2 → 2H2O

Let's say you have 4 moles of H2 and 5 moles of O2.

To determine the limiting reagent, you can compare the stoichiometric coefficients in the balanced equation to see which reactant would be completely consumed first.

For H2: coefficient = 2
For O2: coefficient = 1

To see how many moles of water can be formed with the given amounts of reactants, divide the amount of each reactant by its stoichiometric coefficient.

For H2: 4 moles / 2 = 2 moles of water
For O2: 5 moles / 1 = 5 moles of water

Since you can only produce 2 moles of water from the available H2, it becomes the limiting reagent. Therefore, in this reaction, H2 is the limiting reagent, and O2 is the excess reagent.

To determine the amount of excess reagent remaining after the reaction, you can subtract the moles used by the limiting reagent from the initial moles of the excess reagent.

For O2: 5 moles - 2 moles used = 3 moles remaining

In summary, the limiting reagent is H2, and the excess reagent is O2.

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