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Tuesday, February 22, 2011

Chapter 3. F4: Chemical Formulae and Equations

1. Relative atomic mass and mole of atoms.
a. 1 mole of an element has a mass (in grams)  that is equal to its relative atomic mass.

Atom
Relative atomic mass
Mass of 1 mole (g)
Hydrogen, H
1
1
Carbon, C
12
12
Oxygen,O
16
16
Sodium, Na
23
23
  b. RAM has no unit because it's only a ratio.
 c. if more than 1 isotopes of the element is present, the RAM is calculated by an average of both isotopes's RAM.
d. Why Carbon-12 is used as a standard.
  - its mass is aesily measure with mass spectrometer
  - easily found as its compounds are abundant.
2. Relative molecular mass, RMM
a. A molecule is made up of tho or more atoms. If we know the RAM of all atoms in a molecule, we can calculate the RMM .
b.
Molecule
Molecular formula
Relative molecular mass
Chlorine gas
Cl2
2 x 35.5 = 72
Nitrogen gas
N2
2 x 14 = 28
Ammonia
NH3
14 + ( 3 x 1 ) = 17
Ethanol
C2H5OH
(2 x 12) + (6) + 16 = 46



3.The Mole
 a. the amount of substance is measure by unit of mole
b. the symbol of mole is mol.
c. by using the mole we can calculate the
    - number of particles,  ( x NA).
    - mass of substance , ( x RAM@RMM )
    - Volume of Gas, ( x 22.4dm-3 - STP and x 24 dm-3 at room condition )
4. Chemical Formulae
a. Symbols are used in chemical formula.
b. The formula of molecules tells us -
    -  the name of a substance
    -  the number of atoms of each element present
c.
Compound
Formula
Information
Nitrogen gas
N2
2 atoms of Nitrogen atom in 1 molecule
Hydrogen chloride
HCl
1 atom of hydrogen and 1 atom of Chlorine in 1 molecule of hydrogen chloride
Ammonia
NH3
1 atom of nitrogen and 3 atoms of hydrogen in 1 molecule of ammonia
Sulphuric acid
H2SO4
1 molecule of sulphuric acid contains 2 atoms of hydrogen, 1 atom of Sulphur and 4 atoms of oxygen

5. Empirical Formula and Molecular Formula
 a. Empirical formula only show the simplest ration of elements in it. ( not the actual formula ).
b. Molecular formula shows the actual number of each type of element in it.
c. to determine the empirical formula of MgO
.  - the crucible with its lid are weighed.
  - the crucible together with the lid and Mg ribbon are weighed.
- heated the crucible strongly without lid.
 - when Mg start to burn cover the lid.
 - the crucible's lid is lifted from time to time until the Mg stops burning.
 - the lid is removed and heated strongly for 2 minutes.
 - let the crucible cool down to room temperature and weighed again with the lid.
 - repeat the process of heating , cooling and weighing until a constant mas is obtained.
Observation:
 - metal Mg turns to white solid.
- Precaution measures that's very important to remember:
- sand paper is used to remove the oxide layer
 - the crucible's lid is removed from time to time to allow oxygen enter and oxidised the Mg ribbon.The Lid is then replace quickly to prevent fumes of MgO from escaping.
 - the heating, cooling and weighing process are repeated until a constant mass is obtained.
 - this method also can be used to determine the empirical formulae of reactive metals such as aluminium oxide and zinc oxide.
 d. empirical formula for Copper (ii) oxide ( CuO )
 - fill in 1 spatula of CuO in porcelain boat and weighed then put in the combustion tube.
 - allow dry H2 gas to pass through the combustion tube for a few second, the excess H2 gas is ignites at the other end of combustion tube.
 - the CuO is heated strongly for 10 minutes and let the dry H2 gas to pass through the combustion tube until it cool to room temperature.
- the mass of porcelain boat with Cu is weighed again.
- observation :
 - black colour of CuO turns to brown Cu.
 - Precaution Measures :
 - Dry hydrogen gas is pass through in the beginning of experiment is to remove all the air in the combustion tube.
 - the excess hydrogen gas is ignites is to confirm that all the air is removed. ( if the ignite gas burn without a "pop" sound means that all air is removed ).
 - the hydrogen gas is continuously pass through when the combustion tube is set to cool because to prevent the hot Cu react with Oxygen gas to form CuO again.
- heating cooling and weighing are repeated until a constant mass is obtained.
 -this method also can be used to determine the impirical formula of less reactive metals such as lead(II) oxide and tin(II) oxide.
 6. Molecular Formula
a. shows the actual number of atoms of each element that are present in a molecule of the compound.
       Molecular Formula = ( empirical formula )n where n is a integer.
b. Add all the RAM present , times n = Mass of molecule,
 got the n, then replace in the formula.
7. Percentage Composition by Mass of an element in a compound
 
% composition by   = Total RAM of the element in compound
 mass of element                        RMM of compound

Try to write the chemical formulae and names of the following common compounds.

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