Stoichiometry and Limiting Reagent problems

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slehofer

June 14, 2019 17:51

I was practicing limiting reagent problems. The question I was practicing was:
How many grams what Ag is produced?
19.0g Cu
125g AgNO3

Cu + 2AgNO3 ----> 2Ag + Cu(NO3)2

I discovered that Cu was the limiting reagent and I thought I remembered that we are supposed to use the limiting reagent to determine the moles and then grams of the product. I did so by multiplying the moles of Cu by the mole ratio of 2 moles of Ag for every 1 mole of Cu, giving me 0.598 mol Ag and the resulting mass was 64.5g Ag.

Then, I thought to myself that there is no Ag present in our limiting agent - what difference would I see if I used our excess reagent? So I did the math on converting our given mass of 125g AgNO3 to moles, then converting moles of AgNO3 to moles of Ag, and finally to 79.4g Ag.

That's a difference of 14.9g compared to the mass calculated using the limiting reagent value.

So my questions are:
1. Is it wrong to use the excess reagent in any case to calculate the mass of the product?
2. How does it make sense to use the value of a reactant molecule that doesn't contain any element in the product we are being asked to define?
3. Why is there such a difference between the resulting mass using the values of the limiting reagent and the excess reagent?

Thank you for your time!

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2 comments

von luger

June 16, 2019 00:56
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Firstly, it's wrong to use the excess reagent because how much of it will actually be used in the reaction is completely unknown. Using the limiting reagent is necessary, because that is the one that we know will run out first, so it will be used up entirely in the reaction. Its mass is actually known, while the mass used of the excess reagent is unknown.

Next, Cu or the limiting reagent may not have any part the actual Ag produced, but it does determine how much Ag is freed from the AgNO3. If the Cu is not there, Ag will never be broken away from the silver nitride, and the silver is never produced. So, although Cu is not a part of Ag, it does determine how much Ag is produced.

Lastly, there is such a large difference between masses when using the excess and limiting reagent in this case because of the difference in mass. Just by looking at the initial masses, we see that the Cu mass of 19 g is much smaller than the AgNO3 mass, so that allows us a bit of estimation as to which will run out first. The difference of Ag produced is so big because the using the AgNO3 mass would yield a lot of Ag. However, it must be remembered that not all of the AgNO3 is used, which makes it incorrect to use it to find the mass of Ag produced.
slehofer

June 16, 2019 03:16
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It all makes sense now, thank you!!

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Stoichiometry and Limiting Reagent problems

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