Calculating Molecular and Empirica Formulas

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emilie liu 3G
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Joined: Fri Jul 22, 2016 3:00 am

Calculating Molecular and Empirica Formulas

Postby emilie liu 3G » Sun Sep 25, 2016 5:52 pm

When calculating an empirical or molecular formula, after we convert grams to moles and divide each value by the lowest integer to calculate the mole to mole ratio, how close should we be to a whole number in order to round to the nearest whole number vs multiplying by a factor. For example, if after dividing by the lowest number we receive a value of 3.87, would that be acceptable to round to 4?

Shelby Slaughter 3D
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Re: Calculating Molecular and Empirica Formulas

Postby Shelby Slaughter 3D » Sun Sep 25, 2016 8:34 pm

In the audio visual module Professor Lavelle posted about empirical and molecular formulas he mentioned that a 3.89 would be fine to round up to 4 but a decimal more like 3.75 would signal that you need to multiply by something else (four). I would say 3.87 is close enough to 4 to be rounded. Usually the data in the problems we are assigned will make whether or not we need to round very apparent.

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Re: Calculating Molecular and Empirica Formulas

Postby AnnaTong1E » Sun Sep 25, 2016 9:39 pm

I've read in a few different sources that it is acceptable to round to whole numbers when the number calculated is within one tenth of a whole number. If it is not, the ratio values should be multiplied by some number that will bring all of the values to whole numbers.
That being said, 4.87 is a difficult number to multiply something by to generate a whole number, so in this circumstance I think it would be okay to round up...?

Also I feel like most of the problems should be fairly straightforward in letting you round up or down (meaning the answers will be within one tenth of a whole number) so maybe check your calculations again? Not sure! just an idea

Patrick Ricaflanca 2H
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Re: Calculating Molecular and Empirica Formulas

Postby Patrick Ricaflanca 2H » Mon Sep 26, 2016 4:47 pm

This is an example we did in our discussion session, if you find it helpful.
Ex. Caffeine is 49.48% C, 5.19% H, 16.4% O, and 28.61% N. Find the empirical formula.
1. First, we had to assume it was over 100 g so that we could get rid of the percentages and make them into grams.
2. Then we divide each of them by their molar masses from the periodic table.
49.48/12, 5.19/1, 16.4/16, and 28.61/14
3. Then after you get your values {4.12, 5.11, 1.03, 2} you divide all of that by the smallest number (1.03)
4. The resulting numbers should be whole numbers that correspond to the element in the empirical formula {4,5,1,2}
Answer: C4H5ON2

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Re: Calculating Molecular and Empirica Formulas

Postby aprilhamachi3B » Tue Sep 27, 2016 4:49 pm

Most of the answers I have encountered in the homework have been close enough to round up or down ( ex: 5.02 or 4.98) but I feel that if its fairly off you may have to multiply to get closer to whole number or maybe you messed up somewhere in your earlier calculations. However if you are doing the problem correctly I don't think the textbook would give us a problem with numbers that aren't easy to calculate into a whole number. Overall, trust your judgments with your calculations :)

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Re: Calculating Molecular and Empirica Formulas

Postby usernamesarehard » Fri Sep 30, 2016 10:27 am

Usually, when I find myself off by more than about .2 (or something small but kind of annoying like that), I look back at my work and make any numbers I rounded while showing my work more precise. For example, if I calculated the molarity, I probably rounded that number from 3.94659284639 to 3.95. If I change it back to the long form of the number and do that for any other elements of the final equation, my answer gets closer to the number I thought it would be in the first place thus solidifying my faith in my original answer. It's an easy fix.

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