Finding Molecular Formulas

[Violet Mbela 2B]

When I am working on finding molecular formulas, the process makes sense to me up until you divide all of the mols by the smallest value to find the integers. Because I know that you are multiplying to try to find the smallest possible integer, but what doesn't make sense is how you find the number to multiple all of the mols by. What is that based on/ where does it come from?

[Jenny Han 3F]

I think for me, it is a trial and error process, but I think it also helps to rewrite the ratios as fractions (when more obvious). For example, with the ratio 1:1.33:1 for C3H4O3, 1.33 written as a fraction is 4/3, which makes the whole integer ratio 3:4:3. Hope this helps!

[Sila_Sozeri_1C]

Hi! You divide all the moles to the smallest one to find the ratio between the atoms, which you then use to formulate the empirical formula.

[Ellie Fox 2K]

Hi just adding to the other posts, by dividing by the smallest number, you are just trying to find a whole number ratio between the different atoms.

[Raizel Ferrer 1H]

So when you divide all the moles by the smallest amount of moles, you are finding the mole ratios between the elements. Whenever I see one of the values to be 0.33, 0.66, etc., I know to multiply by 3 in order to make the mole ratios whole numbers for the empirical formula. To get to the molecular formula, you would have to get the known mass of the compound given and divide that by the mass of the empirical formula to get the ratio between the two. That is the number to multiply the empirical formula by to get the molecular formula.

[Daniel Tabibian 3K]

Once you divide, you will have the ratio of each element. You should then multiply by the smallest number which will make them all whole numbers. Rewriting them as fractions may make this easier for you. Once you do this, you will have the empirical formula. To find the molecular formula, calculate the g/mol of the compound and divide that by the g/mol of your empirical formula. The number you get is what you should multiply each element by to get the molecular formula.

[Caitlin_Chheda_1K]

Hi, I think that when you divide all of the numbers by the smallest number, you are left with the mole to mole ratio. Since this typically doesn't result in whole numbers, you have to multiply the numbers in the mole to mole ratio by a whole number. For me, this process is random and takes time, but after doing several practice problems, I started to recognize patterns such as if a number is 1.33 to multiply by 3. From there, when you round, you should have an approximate of the molecular formula. I hope this helps!