December 1st- Nick Kim

 Today we have learned about empirical formula and molecular formula.
First empirical formula is the simplest whole number ratio of  atoms present in compound

How do we get empirical formula?
In order to find empirical formula we need to follow few steps
Let just say you have a compound containing 40% carbon, 53.3% oxygen and 6.7% hydrogen by mass

1,Start with the number of grams of each element, given in the problem. 

If percentages are given, assume that the total mass is 100 grams so that 
the mass of each element = the percent given.

Thus, 40% of carbon is 40g of carbon, and same process for all other elements
40g of carbon
53.3g of oxygen
6.7g of hydrogen

2,Convert the mass of each element to moles using the molar mass from the periodic table
(we have learned  molar conversion last week)
40g of carbon x 1mole/12.0g = 3.3333....... mole of carbon
53.3g of oxygen x 1mole/16.0g= 3.33125 mole of oxygen
6.7g of hydrogen x 1mole/1.0g= 6.7 mole of hydrogen

3,Divide each mole value by the smallest number of moles calculated.
3.3333....... mole of carbon
3.33125 mole of oxygen-smallest
6.7 mole of hydrogen

3.3333....... /3.3333....... =1
3.33125 /3.3333....... =1
6.7/3.3333....... =2

4,Round to the nearest whole number, if number is decimal or fraction.  This is the mole ratio of the elements and is  represented by subscripts in the empirical formula.

Empirical formula: CH2O

here is some practice questions
1, A compound was analyzed and found to contain 13.5 g Ca, 10.8 g O, and 0.675 g H.  What is the empirical formula of the compound?
answer: Ca(OH)2

2,NutraSweet is 57.14% C, 6.16% H, 9.52% N, and 27.18% O.  Calculate the empirical formula of NutraSweet and find the molecular formula.  (The molar mass of NutraSweet is 294.30 g/mol)
answer: C14H18N2O5

Molecular formula
Molecular formula is a multiple of the empirical formula and shows th actual number of atoms that combine to form a molecule

molecular formula= molar mass of the compound/molar mass of the empirical formula

Molar mass of the compund: usually this number is given
Molar mass of the empirical formula: need to use the method that is above

Ex, Molecular formula of a compound is 92.0 g/mol and there are 40g of carbon,53.3g of oxygen and 6.7g of hydrogen

First you need to get emirical formula
do same thing as above information

40g of carbon x 1mole/12.0g = 3.3333....... mole of carbon
53.3g of oxygen x 1mole/16.0g= 3.33125 mole of oxygen
6.7g of hydrogen x 1mole/1.0g= 6.7 mole of hydrogen

3.3333....... mole of carbon
3.33125 mole of oxygen-smallest
6.7 mole of hydrogen

3.3333....... /3.3333....... =1
3.33125 /3.3333....... =1
6.7/3.3333....... =2

Empirical formula: CH2O
 To get molar mass, we simply need to look at periodic table
12.0+(1.0)2+6.0
=20.0g/mol

back to furmula
molar mass of the compound/molar mass of the empirical formula
92.0 g/mol/20.0g/mol
=4.6
4.6(CH20)
usually there arent decimal. Sorry lol

here are some videoes that will help
http://www.youtube.com/watch?v=r2Log6-voWo
http://www.youtube.com/watch?v=nslC7lOSc7Y