### Empirical Formula

Posted:

**Mon Sep 30, 2019 8:18 pm**When using the empirical formula do we always assume that the sample mass is 100g?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=11&t=45745

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Posted: **Mon Sep 30, 2019 8:18 pm**

When using the empirical formula do we always assume that the sample mass is 100g?

Posted: **Mon Sep 30, 2019 8:23 pm**

Yes, when we are provided with the percentage mass compositions it is best to assume a sample mass of 100g so that the percentages can be directly converted into grams. For instance, if you had a compound that was 35.7%Al and 64.3%Cl, assuming a sample mass of 100g, we could convert those percentages into 35.7g Al and 64.3gCl and continue the problem from there.

You do not have to use 100g, however. For instance, if you used 50g, then you would convert the percentages into masses by finding out what numbers are 35.7% and 64.3% of 50g respectively. But since it allows us to more directly convert the percentages into masses, assuming 100g is much simpler. This is why Professor has suggested we assume a 100g sample mass. I hope this helps!

You do not have to use 100g, however. For instance, if you used 50g, then you would convert the percentages into masses by finding out what numbers are 35.7% and 64.3% of 50g respectively. But since it allows us to more directly convert the percentages into masses, assuming 100g is much simpler. This is why Professor has suggested we assume a 100g sample mass. I hope this helps!

Posted: **Mon Sep 30, 2019 8:23 pm**

You only assume that the sample is 100g to make the calculations easier. For example if a CH4 sample contains 20% carbon but the sample itself is only 10 grams. You could assume for a second that the sample was 100 grams and 20 grams of that was carbon. Then you could use that ratio to say a 10 g sample of CH4 contains only 2 grams of Carbon.

Posted: **Mon Sep 30, 2019 8:26 pm**

You only assume that the sample mass is 100g to make the calculations easier. Because percentages are on a scale of 1-100, assuming that the sample mass is 100g allows you to skip a step and just convert the percentages to masses by changing the units from a percent sign to grams.

Posted: **Mon Sep 30, 2019 8:27 pm**

When given the mass percents of atoms in a compound, it is easiest to assume a 100g sample because then you can simply convert the percentages to masses. It is not necessary to assume a 100g sample due to the fact that the mass percents of each atom will remain the same regardless of the sample mass. However, if we are told that a sample has 30.0% NaCl, then with a 100g sample, we can start the problem by converting 30.0g NaCl to moles and then following the typical procedures of finding empirical formulas. If you were to assume a 200g sample, then the starting mass would be 60.0g NaCl. You would still end up with the same empirical formula, but assuming a 100g sample is the most convenient way to begin a problem when the sample size is not given.

Posted: **Mon Sep 30, 2019 8:30 pm**

You don't have to assume that it is 100g, however, it is easier to assume that it is, because you want to convert the mass percentages into actual sample masses, and then use that to find out the molar mass of each element. 100g is a good, nice number, and assuming a 100g sample makes it easier to do your calculations.

Posted: **Mon Sep 30, 2019 8:41 pm**

It isn't necessary to say the mass is 100g. It just makes the calculations much easier because we use mass percentage composition and percentages are 100.

Posted: **Mon Sep 30, 2019 10:18 pm**

I learned this simple trick in my high school Chemistry class.

1. Percent to mass (change the percent sign into a grams sign)

2. Mass to Mole (Convert mass to moles using molar mass)

3. Divide by Small (divide by the smallest number)

4. Times to whole (times to get whole numbers)

1. Percent to mass (change the percent sign into a grams sign)

2. Mass to Mole (Convert mass to moles using molar mass)

3. Divide by Small (divide by the smallest number)

4. Times to whole (times to get whole numbers)