The molar concept.

We worked more on moles to better our understanding on this very fascinating subject.
A mole is 6.02 x 1023. If you're having trouble remembering this then there is great song that can help you with it, the link will be at the bottom. 

          Ar and Mr        

Ar is the relative atomic mass, and this simply means the average mass of one type of an element to one twelfth of the mass of an atom of carbon-12 and this has no units. For e.g, the relative atomic mass for oxygen is 16. 

Mr is the relative molecular mass, is the weight of all the atoms of a molecule added together. This can be calculated. 

Find the molecular mass of HCL.

H- 1

Cl- 35.5

35.5 + 1 = 36.5. therefore the molecular mass of HCL is 36.5

Percentage compositions. 

The percentage composition indicates how much of an element there is in a compound. This can be done by dividing the relative atomic mass of the element and dividing it by relative molecular mass. 

Which fertilizer, urea(COCNH2)2) or ammonium nitrate (NH4NO3) contains the most nitrogen? 

(14x2)/ (12+16+32) x 100 = 46% or (14x2)/(28+4+48) x 100 = 35% 

therefore the answer is Urea. 

Empirical and molecular formulas. 

The empirical formula of a compound is the simplest ratio of the atoms it contains. It must still be stated in whole numbers though. For e.g

  what is the empirical formula of C3H6?

 the empirical formula would be CH2

The molecular formula on the other hand is the same as or a multiple of the empirical formula, and is based on the actual number of each type in the compound. To find the molecular formula, divide the mass of the element by the atomic mass. 

"2.3g of sodium burn't in air to form 3.1g of an oxide, what is the formula of the oxide?"

2.3/23 = 0.1

(3.1-2.3 = 0.8)

0.8/16 = 0.05

2:1 therefore Na2O

To put what we learnt into application, we did a lab on finding the formula of a hydrate.

Today, we learned about the empirical formula and the molecular formula and how to find the molecular formula. We also learnt how to calculate percentage compositions and the difference between relative atomic formula and relative molecular formula. 

Mole song: http://youtu.be/PvT51M0ek5c

pictures: http://www.revisionworld.com/sites/revisionworld.com/files/rw_files/atomicmass.jpg

http://antoine.frostburg.edu/chem/senese/101/moles/slides/img009.GIF

http://img.docstoccdn.com/thumb/orig/78182635.png