## Calculating Molecular and Empirica Formulas

emilie liu 3G
Posts: 23
Joined: Fri Jul 22, 2016 3:00 am

### Calculating Molecular and Empirica Formulas

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
Posts: 24
Joined: Fri Jul 15, 2016 3:00 am

### Re: Calculating Molecular and Empirica Formulas

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.

AnnaTong1E
Posts: 20
Joined: Fri Jul 15, 2016 3:00 am

### Re: Calculating Molecular and Empirica Formulas

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
Posts: 21
Joined: Sat Jul 09, 2016 3:00 am

### Re: Calculating Molecular and Empirica Formulas

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}

aprilhamachi3B
Posts: 9
Joined: Wed Sep 21, 2016 2:58 pm

### Re: Calculating Molecular and Empirica Formulas

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 :)