LIMITED REAGENT: it is the reagent that determines the amount of product that can be formed by a reaction. The reaction occurs only until the limited reagent is used up.
The second term is EXCESS REAGENT: the unlimited amount of reactant. But the term is self explanatory.
In our exercises we learnt how to identify the reagents.
For example: using the analogy of recipes there can only be a limited amount of sandwiches produce with 4 slices of bread it is known to be the limited reagent.
Practice #1
Information:Copper reacts with sulfur to form copper (1) sulfide according to the following equation: Atomic Mass
2Cu+S-->Cu2S Cu=63 S=32
What is the limiting reagent when 80g of Cu reacts with 25g of S?
Step #1: If 1 mole of Cu (copper) weighs 63, how many moles are there if you have 80g
your equation should look something among the lines of this
1 MOLE of Cu = 63g using cross multiplication
X MOLES of Cu = 80g
X = 1.269 or 1.27 moles
Step #2: If one mole of S (Sulfur) weighs 32g, how many moles are there for 25g
your equation should look something like this
1 MOLE of S=32g using cross multiplication
X MOLES of S=25g
X=0.781 moles
Step#3: In this step you will be able to compare the amounts and quantities
If there is one mole of Sulfur and gives you 2 moles of Cu as a product, you will be able to calculate the end result with different amounts of moles like this:
1S--->2Cu Cross multiplication 1S--->2Cu
0.78moles of S --->X X moles--->1.27
X= 1.56 moles X=0.635 moles
Step #4: answer the question by comparing the amount of moles from the 80g of Cu (1.27) to the result you got from step #4, and depending on the contrast, in this case the answer from step #1 is less than 1.56 than your are able to say that
Answer: 1.27 moles of Cu is your limited reagent
0.78 moles of S is your excess reagent
Your process you do is not obligatory to do the same, it can be recommended, but the best method is the one you understand.
Stefano Pinzon
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