Limiting Reagents [ENDORSED]

[itzeelpadillaDis1A]

what are other ways a problem can ask to find a limiting reagent without actually saying "find the limiting reagent" ? (not actually saying the term "limiting reagent" but using an alternate way to ask the question)

[Thuy-Anh Bui 1I]

Generally, if the problem gives you an amount (in grams or moles) of each reactant to make a calculation, you are implicitly asked to find the limiting reactant. A good example of this is question M.7 (b), which asks "What mass of boron can be produced when 125 kg of boron oxide is heated with 125 kg of magnesium?" You have no way of knowing which value (125 kg of boron oxide or 125 kg of magnesium) to use to calculate the mass of boron unless you determine which one is the limiting reactant.

[Nina Gautam 1K]

Another way a question could be implicitly asking about limiting reagents is by asking what the maximum or theoretical yield is from a given set of masses (of reactants), or by providing masses of reactants and the actual yield of an experiment and requiring us to find the percent yield.

[Gabriela Carrillo 1B]

For problem M.9 what's the process to figuring out how to write the net ionic equation for the reaction? Also will we need to know how to do that by the quiz this friday?

[Kyung_Jin_Kim_1H]

For M9 (and other net ionic problems), first write the complete chemical equation. Then rewrite the chemical equation, except this time writing down ions that separate (ex. salts like NaCl, strong acids like HCl). Cancel out any ions that are on both sides.

I don't think we have to know this for the test on Friday though, because it wasn't covered in the modules or during these past few lectures.

[Lauryn Jordan 1F]

It could also ask instead of limiting reactant, which one is the excess and how much excess is there? Or it could ask how many moles can be produced?

[Wenxin Fan 1J]

Problems might ask for a reactant that is in excess. Also, when working a problem with a given amount (either in moles or in grams) of each reactant, you should always check to see which is limiting in order to determine the theoretical yield. Many problems won't explicitly ask you to identify a limiting reactant but you should always be aware that you might need to find it to answer the question.

-Wenxin Fan 1H

[ZoeHahn1J]

The problem could ask you to:
- find the percentage yield
- find the theoretical yield (in moles or grams; "how much product in moles/grams is expected to form from x amount of reagent?")
- find the reagent that is in excess
Since all of these will require first finding the limiting reactant.
Hope this is helpful!

[Chem_Mod]

A lot of good student group interaction. Great posts.
Hopefully Monday I'll remember to comment on this in class. :-)