### Formulas

Posted:

**Thu Jun 27, 2019 12:17 pm**Can someone please explain how we find the number to multiply by when making the molecular formula?

Created by Dr. Laurence Lavelle

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

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=11&t=45323

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Posted: **Thu Jun 27, 2019 12:17 pm**

Can someone please explain how we find the number to multiply by when making the molecular formula?

Posted: **Thu Jun 27, 2019 12:53 pm**

In order to find the molecular formula, the actual molar mass of the substance must be given in the problem. If that's not given, you don't solve for the molecular formula. If so, divide that value, which should be g.mol^-1, by the molar mass computed by adding up the atomic weight of the elements in the substance. After, you use the quotient(should be a whole number) of that division and multiply that by each of the subscripts in the empirical formula. Hope this helps.

Posted: **Fri Jun 28, 2019 10:33 am**

As previously stated the molecular formula can not be solved without knowing what the molar mass is. If you have the molar mass then you can divide the molar mass by the molar mass of the empirical formula, if you calculated the empirical formula. Now if you have the molar mass of the molecule and the percentages of each element then you can multiply the percentages by the molar mass to get the # of moles of each element in the molecular formula. If the question gives you the sample mass and the mass of each element in the sample you can calculate the percentages and then use the molar mass to find the # of moles of each element in the molecular formula as well. Itâ€™s important to note that without the molar mass you can not find the molecular formula, but if the question asks to find the molecular formula it will most likely be given.

Posted: **Sat Jul 06, 2019 6:08 pm**

You first divide by the smallest mole then multiply the number that will make the empirical formula into all whole numbers. After that, you use the grams/mol that is given and divide the molar mass by that to see what is the factor that your moles have to be multiplied by.

Posted: **Sun Jul 21, 2019 1:54 pm**

After you get the empirical formula, you get the molar mass of the empirical formula of the compound. If you need to find the molecular formula, the question will give you the molar mass of the molecular formula. Now you divide the molar mass of the molecular formula by the molar mass of the empirical formula, you get the number to multiply by.