Trouble in finding the Molecular Formula

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Juan Liberato
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Joined: Wed Sep 21, 2016 2:59 pm

Trouble in finding the Molecular Formula

Postby Juan Liberato » Sun Jul 02, 2017 9:36 pm


I had a question concerning a step on finding the molecular formula.

Once we find the mols of all the elements such as in the Vitamin C example problem and we must divide by the smallest value how do we go from there?

In the Course Reader I am aware you multiply by 3 to get all the numbers as close to a whole number but I am having difficulty doing it with any other problem.
In the example, two elements are 1:00 but what do we do in the scenario that it is 1:00 and 1:33 and a number like 1:57 or any other number.

Please and thank you!

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Joined: Fri Jun 23, 2017 11:39 am

Re: Trouble in finding the Molecular Formula

Postby plopezcordon_4C » Sun Jul 02, 2017 9:41 pm

I think you multiply the mole ratios by 2 or 3 to get the whole number (because there are never fractions). Then you use the new mole ratio to get the empirical formula. I think that's right.

Giselle Garcia 1B
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Joined: Wed Sep 21, 2016 2:58 pm

Re: Trouble in finding the Molecular Formula

Postby Giselle Garcia 1B » Sun Jul 02, 2017 9:56 pm

Hey Buddy!
So to answer your question, after you divide by the smallest number, each element will have their molecular formula's number and multiplied by their atomic mass. Like if the ratio of CHO is 3:4:3 then you would multiply Carbon's atomic weight (12.01) and multiply that (3), Hydrogen (1.008)x(4), Oxygen (16)x(3). After so, you then add them up to get Molar mass of the empirical formula unit (88.06 g/mol^-1) and use that to divide the molar mass of the compound (176.12 g/mol^-1) and you would get 2. Once you get that, you would multiply that by molecular formula to get your answer (which is C6 H8 O6). I hope that helps!


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Re: Trouble in finding the Molecular Formula

Postby kaushalrao2H » Wed Oct 04, 2017 5:17 pm

Sometimes, when the molar ratios aren't as clean as 1:1:1.33, you're going to have to guess and check and see what coefficient you need to multiply the molar values by.

Morgan Baxter 1E
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Joined: Thu Jul 27, 2017 3:00 am

Re: Trouble in finding the Molecular Formula

Postby Morgan Baxter 1E » Wed Oct 04, 2017 6:09 pm

Once you have the ratio in decimal form such as 1:1:1.333, in some instances it could be helpful to convert the decimals to fractions. Then the ratio is 1:1:4/3. Written like this, it is easier to see that multiplying by three would get rid of all the denominators and turn into a ratio of whole numbers 3:3:4.

Christy Zhao 1H
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Joined: Fri Sep 29, 2017 7:07 am

Re: Trouble in finding the Molecular Formula

Postby Christy Zhao 1H » Wed Oct 04, 2017 11:58 pm

I understand that we have to multiply the mole ratios to get as close to whole numbers as possible, but to what extent would the decimal be considered too far from a whole number? For example if I get 1.86, would I still be able to round that to 2?

Aliza Ajmal 1D
Posts: 25
Joined: Fri Sep 29, 2017 7:05 am

Re: Trouble in finding the Molecular Formula

Postby Aliza Ajmal 1D » Thu Oct 05, 2017 10:35 pm

If the molar ratio is 0.1 off then you can round up. For example, if the number is 1.9, then it can be rounded up to 2.0. If the number is 1.86, even rounding up seems to be fine, but if you'd like to be safe you can multiply it by the number 6 to get a closer whole number. Honestly, I think as long as you multiply all the numbers in the ratio by the same number it should be fine. The empirical formula is just the ratio of the atoms after all, and isn't the actual total number of atoms. Also, when you go on to finding the molecular formula through the molar mass, it won't be much different in coming to an answer.

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