Empirical Formula and Molecular Formula

The empirical formula of a chemical compound is the simplest positive integer ratio of atoms of each element present in a compound. Using the ration you can identify the name of an unknown compound. In this blog i am going to demonstrate my understanding on how to use this formula. Lets start with some problems:

1. You have 70% of Fe(Iron) and 30% of O(Oxygen) is the compound FeO or Fe2O3.
1. Assume that one rock of Iron Oxide is 100g.
1. you can than say that there is 70g of Fe and 30g of O
2. next is to find the number of moles in 70g of Fe and the number of moles in O
1. 70gFe=              mol/Fe, 30gO=             mol/O
3. to figure this out you would look at the atomic mass of each element
1. Fe has an atomic mass of 56 and O has an atomic mass of 16
2. taking each atomic mass you divide by the number of grams
1. 70/56=1.25, 30/16=1.875
3. the answer will be the number mole in each element.
1. 70gFe=  1.25  mol/Fe, 30gO=   1.875  mol/O
4. you than take the lower number in this case 1.25 and divide 1.875 and 1.25 by 1.25
1. 1.25/1.25=1, 1.875/1.25=1.5
2. When you get a decimal like 1.5 you will multiply both answers by two
1. 1.25/1.25=1x2=2, 1.875/1.25=1.5x2=3
5. you than have your ration for every two iron there are three oxygen
1. 2:3

This next problem I will not only use the empirical formula but to find the right answer when given to compounds that can work for the ratio I will than use the Molecular Formula.

Empirical Formula part 1

1. YOu have 2.34g of N(Nitrogen) and 5.34g of O(Oxygen) you need to find out if this compound is No2 or N2O4? Weight is 92g/mol
1. First find the number of moles in each
1. 2.34gN=      mol/N, 5.34gO=      mol/O
2. the atomic mass of N is 14 while the atomic mass of O is 16 divide the grams by the mass
1. 2.34g/14=0.167, 5.34/16=0.334
3. 2.34gN= 0.167  mol/N, 5.34gO=  0.334  mol/O
1. divide by the lowest number(0.167)
1. 0.167/0.167=1, 0.334/0.167=2
4. you have a ratio of 1 nitrogen for every 2 oxygen
1. 1:2
5. Now find the weight simple add 16x2+14=46g/mol the weight doesn't match what we were given.
2. Find out if the compound is N2O4 we have to use the molecular formula. You are given the weight of 92g/mol to get the answer you simply do the following.
1. n x46=92g/mol
1. 2x47=92g/mol
1. answer is N2O4