Wednesday, November 18, 2009 4:36 PM
EMPIRICAL FORMULAS!
The difference between a molecular formula and an empirical formula is that the molecuar gives you the actual number; for example: Pb2O4. An empirical formula however, gives you the number ratios of the elements in the compound; Pb2O4 would become PbO2.
Example:-What is the empirical formula for 15.9% of B and 84.1% of F?
Step 1: create a chart like the following:
ELEMENT: B
MASS (g): 15.9
ATOMIC MASS: 10.8
MOLES: 15.9/10.8= 1.5 moles
MOLES/SMALLEST MOLE: 1.5/1.5= 1
RATIO: 1
ELEMENT: F
MASS (g): 84.1
ATOMIC MASS: 19.0
MOLES: 84.1/19.0= 4.4 moles
MOLES/SMALLEST MOLE: 4.4/1.5= 2.93333
RATIO: 3
The empirical formula is BF3.
Step 2: once you have set up the chart you start to fill it in. the elements are Boron and Fluorine, their masses are 15.9 g and 84.1 g, their atomic mass is 10.8 and 19.0, and they have 1.5 moles and 19.0 moles. when you get to the moles/ smallest mole section you simply look at the number of moles each element has B= 1.5 mol, and F= 4.4 mol. boron has the smallest mol, therefore you divide both moles by 1.5. if you do not get a whole number then you either round it or multiply it by 1, 2, 3, 4 etc... until you get one. but remember if you multiply one of them then you have to multiply the rest by the same number.
Go to this site to see a video:
http://www.youtube.com/watch?v=r2Log6-voWo