Determining Limiting Reactant by Mole or Mass

[Vanessa Chuang 4F]

In lecture I noticed that Dr. Lavelle usually determines the limiting reactant by comparing calculated moles to required moles. I was taught in high school to convert all moles to masses first and then do the comparison. In that case, does either method work? If not, then why is there a distinction? Thanks!

[Brian_Ho_2B]

In lecture I noticed that Dr. Lavelle usually determines the limiting reactant by comparing calculated moles to required moles. I was taught in high school to convert all moles to masses first and then do the comparison. In that case, does either method work? If not, then why is there a distinction? Thanks!

Personally, I was taught in high school to compare the molar ratios by converting all masses to moles. Converting masses to moles is necessary because that is the only way to effectively use the balanced equation to see how much "x" amount of moles of reactant will produce "y" amount of moles of a product. This is the overall idea of stoichiometric calculations. You want to take whatever info you have (eg. mass of reactants), convert those numbers to moles, and then use dimmensional analysis as well as the balanced chemical equation to calculate how much product could be produced by reactant A and reactant B (in terms of moles). Once you have the moles of the products calculated, you can convert that to mass if the question asks for it. In conclusion the best way to approach stoichiometric problems can be summarized in two short visuals:
1. mass reactant --> moles reactant --> moles product --> mass product
or if the question asks for necessary mass of reactant A to produce product B
2. mass product --> moles product --> moles reactant --> mass reactant
I hope this helps! If you're still unsure I recommend checking out a youtube channel called TheOrganicChemistry tutor. His videos are very straightforward but have complete explanations.

[Jasmine 2C]

Honestly, my high school chemistry background is quite terrible but in my opinion, I think both way definitely works since you can always convert between mass and moles. But I strongly agree with the above answer. Using moles is very efficient when using the balanced equation because you can see the molar ratio of reactant A to reactant B (or reactant A to product C, and so forth). Also, since most of the questions I've encountered so far ask to find how much of a certain reactant or product is needed in grams, it would just make your life easier to start with the number of moles they already provided and eventually convert to mass in the end.
Hopefully, my answer didn't confuse you even more and actually helped!

[904914037]

I also personally convert the masses of the reactants to moles and to determine which is limiting I multiply the the amount of moles of one reactant by the molar ratio of the reactants to determine how much of that reactant will be used in the reaction, thus revealing whether it is limiting or not. For example, consider the balanced equation 2Al +3Cl2->2AlCl3, where there are 9.45 moles of Al and 7.55 mol of Cl2. I would multiply 9.45 moles Al by the molar ratio of Cl2 to Al which is (3molCl/2molAl) to find that the reaction requires 14.2 moles of Cl2. Since we only have 7.55 mole Cl2, Cl2 is the limiting reactant.

[Veronica_Lubera_2A]

I asked this same question to one of the TA's (I also was taught the same way) and he said that it should be fine as long as it makes sense and you get the correct answer. Remember in chemistry there are multiple ways to solve one problem so as long as it uses correct reasoning, there should be no issues.

[Ashley Kim 3F]

I agree that there are multiple ways to go about chemistry problems, but from my experience, what the limiting reactant is is clearer when you are comparing moles. Only looking at the given masses of molecules will not always tell you the limiting reactant because you need to take into account the molecular mass of the molecule, but once you find the molecular mass, you can use ratios to figure out the limiting reactant!

[Megan Jung 3A]

Either way is correct in determining the limiting reactant. If you convert all of the products to moles, then you should compare the molar ratios using stoichiometric coefficients to determine which reactant is the limiting reactant. However, if you choose to convert the products to grams, you are adding an extra step by multiplying moles by the molar mass of the product. After determining the mass for the products you can determine the limiting reactant by comparing the amount (grams). Whichever reactant creates the least amount of product (in grams) will be the limiting reactant.

[Timmy Nguyen Dis 1I]

In my opinion, it is easier to use moles, because you can just compare it to the chemical reaction.

[Amanda Mei 1B]

I learned to convert moles of reactants to mass of a product and compare which reactants created less/more product. Both ways work. Using Lavelle's way of comparing calculated moles to required moles saves you a step, but I found that problems often asked for how much product would be created, so I ended up having to do the extra step anyway.