Limiting Reactant Steps General Question

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Michelle Cervantes 1H
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Joined: Fri Sep 28, 2018 12:18 am

Limiting Reactant Steps General Question

Postby Michelle Cervantes 1H » Wed Oct 03, 2018 10:42 pm

I know at some point, which I think is calculating the molar mass, we envision 100g of the reactants in order to find the moles but I am confused as to why we see it that way. I believe this is during the step where we convert from masses to moles. Thanks

Catherine Kim 3K
Posts: 31
Joined: Fri Sep 28, 2018 12:26 am

Re: Limiting Reactant Steps General Question

Postby Catherine Kim 3K » Wed Oct 03, 2018 10:48 pm

If you add the mass percentages of each element in a formula together, it usually equals around 100% (does not go over though). Therefore, you can assume 100g of the sample.

Rachel Yoo 1F
Posts: 65
Joined: Fri Sep 28, 2018 12:24 am

Re: Limiting Reactant Steps General Question

Postby Rachel Yoo 1F » Wed Oct 03, 2018 10:51 pm

When finding empirical or molecular formula, its important that we know the mass percentage of each element. For example, we find the mass percentage is H 4.58%, O 54.5%, and C 40.9%. Since these percentages all equal to 100% it is the easiest procedure to imagine the sample mass being 100g because we already know that H is 4.58 of 100, O is 54.5 of 100, and C is 40.9 of 100. That is why we use 100 g because it is already set up so we don't need to do extra calculations.

Srikar_Ramshetty 1K
Posts: 64
Joined: Fri Sep 28, 2018 12:27 am

Re: Limiting Reactant Steps General Question

Postby Srikar_Ramshetty 1K » Wed Oct 03, 2018 10:55 pm

Great question! We use an "imaginary" value of 100g to simplify calculations that need to be made to isolate the amount of moles of a certain element when we are dealing with mass percentages. Since percentages are proportional it does not matter what the mass we use is. This is also true for the relationship between the amount of moles for each element, since the end goal is to create an empirical/molecular formula. You could just as easily use 200g value or even 50g, but we use 100g since percentages run on the range of 0 to 100 and forms a 1%:1g ratio, which is very beneficial when the mass percent value is a whole number.

Hope that helped, feel free to ask for clarification if it didn't!
- Srikar Ramshetty


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