Empirical Formula: Gives the lowest whole number ratio of atoms in a the formula.
Molecular Formula: A multiple of the empirical formula that shows the actual number of atoms that combine to form a molecule. (I will show you how to determine what the multiple is later on.)
Let's being with the empirical formula...
What is the empirical formula of a compound consisting of 60.0% C and 40.0% H?
We are going to assume that we have 100g of the compound, because 80% + 20% works out to 100%. It is the ratio that counts, so it's okay if we don't really have 100g- the ratio will still be the same as long as the percentages are the same.
1) Find the number of moles of each element...
80g of C x 1mole of O
-------------- = 6.67 moles of oxygen
16.0g
20g of H x 1 mole of H
--------------- = 20 moles of H
1.0g
2) Now divide each of the values of moles by the smallest value of mole that you've calculated. The number that you get will be that elements ratio.
6.67
For Oxygen: -------- = 1
6.67
20
For Hydrogen: ------ = 3
6.67
Therefore, your empirical formula is CH3.
If you ever get ratios that are like 2.50 or 5.66, just multiply all the ratios by a number that will give you a whole number. For example, if you had:
5.66
3
1
You would multiply each of those ratios by 3 because that would give you whole numbers:
5.66 x 3 = 17
3 x 3 = 9
1 x 3 = 3
Voila!
Now moving on to molecular formula...
It requires you to find the empirical formula, then multiply it by a multiple.
To find that multiple, you use this formula:
molar mass of compound
multiple = -------------------------------------------
molar mass of the empirical formula
A molecule has an empirical formula C2H5, and a molar mass of 58g/mol.
Molar mass of empirical: (2x12)+(5x1) = 29g/mol
Molar mass of compound: 58g/mol (given to us in the question)
58g/mol
------------- = 2
29g/mol
Now we know that our multiple is 2, so we need to multiply our empirical formula by 2.
2(C2H5) = C4H10
C4H10 is our molecular formula.
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