Question about Molar Ratios with Limiting Reactants

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Isabelle Hales 1J
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Question about Molar Ratios with Limiting Reactants

Postby Isabelle Hales 1J » Wed Oct 07, 2020 3:20 pm

I understand the basics of identifying a limiting reactant. However, I am a bit confused on how it works in certain cases when you have a balanced equation with different stoichiometric coefficients.

For example: 1 A + 2 B = 2 C + 1 D

Let's say you are going through this problem and find that reactant B has less moles than reactant A. Would you need to double the moles of B before determining if it is the limiting reactant because it has the stoichiometric coefficient of 2? If then reactant A has less moles, would it now be the limiting reactant?

Sydney Lam_2I
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Re: Question about Molar Ratios with Limiting Reactants

Postby Sydney Lam_2I » Wed Oct 07, 2020 3:23 pm

Hi!

So what I do is I find the amount of moles of each reactant and then I divide by the stoich coefficient that the molecule has in the balanced equation. From there I compare the moles and see which one is the smallest. Hope that helps!

Faith Veenker 2K
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Re: Question about Molar Ratios with Limiting Reactants

Postby Faith Veenker 2K » Wed Oct 07, 2020 3:24 pm

I believe your explanation at the end is correct. Once you find the moles of molecule A and B, you need to determine how many moles of the other reactant each one require. For example, if you have 4 moles of A and 6 moles of B, B would be the limiting reactant because you need to multiply moles of A by 2 (because each mole of A needs 2 moles of B).

HannahRobinson3L
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Re: Question about Molar Ratios with Limiting Reactants

Postby HannahRobinson3L » Wed Oct 07, 2020 3:27 pm

In this case, if reactant B has less moles than reactant A, then reactant B would still be the limiting reactant because for every one mole of A, 2 moles of B need to be used. This means that you would actually need more than double the moles of reactant B for reactant A to be the limiting reactant. If you were doing a problem with these numbers, after you converted the grams of each reactant to moles, you would actually divide each by the stoichiometric coefficient and then see which one had the smaller amount.

Faith St Amant 3D
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Re: Question about Molar Ratios with Limiting Reactants

Postby Faith St Amant 3D » Wed Oct 07, 2020 3:30 pm

The way that I usually approach problem solving examples like yours is I'll find how many moles of each reactant are present, and then I'll multiply whichever reactant that has the smallest stoichiometric coefficient in the balanced equation by the other coefficients to determine which is the limiting reactant. In your example, you determined the number of moles present of both A and B, so I would then go ahead and multiply the number of moles of A by 2, and compare that value with how many moles of B are actually present. If 2 times the moles of A is larger than the number of moles of B present, then B is the limiting reactant and vice versa.

AnjikaFriedman-Jha2D
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Re: Question about Molar Ratios with Limiting Reactants

Postby AnjikaFriedman-Jha2D » Wed Oct 07, 2020 3:31 pm

Once you determine the moles of reactants, you have to multiply by the mole ratio of products: reactants in order to find the theoretical yield of any particular product, you always want to have the moles of product in the numerator so you cancel out the moles of reactant in the denominator

Jiapeng Han 1C
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Re: Question about Molar Ratios with Limiting Reactants

Postby Jiapeng Han 1C » Wed Oct 07, 2020 9:48 pm

You must have 2 units of B and 1 unit of A in order for the reaction to proceed. So if you have fewer moles of B than A, A is then clearly in excess. What you really need to do is doubling the mole of A to see if it equals number of moles of B. If it is more than moles of B, than B is the limiting reactant; if it is fewer, then A is the limiting reactant.

Brendan Duong 1I
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Re: Question about Molar Ratios with Limiting Reactants

Postby Brendan Duong 1I » Wed Oct 07, 2020 11:49 pm

For every 1 A you need 2 B in order to get 2 C and 1 D. If you don't have at least1 for every 2 B, A is the limiting reactant. If you don't have at least 2 B for every 1 A, B is limiting. It's kinda like saying for every 1 motorcycle engine, you need 2 wheels in order to have the end product of a whole motorcycle.

Yuehan_Wu_3K
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Re: Question about Molar Ratios with Limiting Reactants

Postby Yuehan_Wu_3K » Thu Oct 08, 2020 5:11 am

Hi so My advice for you is to calculate the number of the other reactants using the given numbers and compare. for example, if the number of A, and B is coorespondingly 0.5 mole and 2 mole. Considering A firstly, the 0.5 mole A is using 1 mole B, which is less than the number of B given. Then, the 2 mole B will use 1 mole A, which exceed 0.5 mole. If the calculated number is less than the given number, then the base reactant is the limiting reactant.


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