Thursday, December 2, 2010

Empirical & Molecular Formula

Hi everyone! This is Melissa, and today I will be guiding YOU- yes you- through the steps of.... determining the empirical and molecular formula of a compound!

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

20g of H x 1 mole of H
                                        --------------- =  20 moles of H

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.

 For Oxygen:      -------- = 1

      For Hydrogen:  ------ = 3

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:


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


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)

      ------------- = 2

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.

With practice, finding molecular and empirical formulas will become less confusing!

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