Formula Weight (FW) (units = amu)
The atoms displayed in the formula constitutes one
FORMULA UNIT
The formula weight = mass of 1 formula unit in amu’s.
This is simply the sum of the AW’s of the atoms displayed
in the chemical formula.
Example: H2SO4 sulfuric acid
2 hydrogens
1 sulfur
4 oxygens
FW (H2SO4) = 2AW(H) + AW(S) + 4AW(O)
= 2(1.0 amu) + (32.0 amu) + 4(16.0 amu)
= 2.0 amu + 32.0 amu + 64.0 amu
= 98.0 amu
Molar mass H2SO4 = 98.0 g/mol
Molecular Weight (MW) (units = amu)
Common term for formula weight of molecular
(covalent) substances.
Often used interchangeably for the term formula weight for
all substances. This is not strictly proper for ionic
substances which have no molecules.
Molar Mass
Equals the mass of 1 mole of formula units of a substance.
(6.02 x 1023 formula units)
Molar mass of a molecular or ionic substance is
numerically equal to its formula weight.
The units, however, are grams per mole (g/mol)
Formula Weight Type of Type of Molar Mass
(amu) Weight substance
(g/mol)
Na
22.99 AW Monatomic
22.99
element
H2
2.02 FW or Diatomic
2.02
MW
element
H2O
18.02 FW or Molecular
18.02
MW Compound
FW
Mg(OH)2 58.33
Ionic
58.33
Compound
When calculating molar masses use as many
significant figures as your periodic chart allows.
Follow the rules for significant figures.
Percent composition
Refers to mass% of each element in a compound.
% Element

Atoms of Element AW 

100
FW of Compound
Example:
Calculate the percent oxygen in CO2
Formula weight of carbon dioxide:
1(12.01 amu) + 2(16.00 amu) = 44.01 amu
Percent Composition of oxygen:
2(16.00 amu) x 100
44.01 amu
= 72.71 % O
Percent Composition of carbon = 27.29% C
Empirical Formulas from Chemical Analysis
If the percent by mass of the elements is known,
the empirical formula (simplest ratio) can be found.
Follow the flowchart page 121.
Example:
Percent composition analysis shows for a compound:
Hg  73.9%
Cl  26.1%
Convert percents to grams. This is how many grams of
each that there would be in a 100g sample.
Hg  73.9 g
Cl  26.1g
Next convert grams to moles using the molar mass of
each element.
Hg
 1mol 
  0.367mol
mol  73.6 g 
 200.59 g 
Cl
 1mol 
  0.736mol
mol  26.1g 
 35.45 g 
Lastly, determine the smallest whole number ratio of
moles of the elements to give the empirical formula.
Do this by dividing the moles of each element found in
the previous step, by the smallest number of moles
found.
Convert to whole numbers if you get a fraction.
Hg 0.367 Cl 0.736
0.367
0.367
Hg1Cl2
Finding molecular formulas from empirical formulas
Molecular formula is a whole number multiple of the
empirical formula. (Multiplier for the subscripts)
To find the molecular formula, you must know the
molecular weight(MW) and the emipirical weight(EW).
Empirical weight = sum of AW’s of atoms shown in the
empirical formula
Whole number multiple = MW/EW
Example
Empirical formula: CH
Empirical formula weight: 13.019 g/mol
Known Molecular weight: 78.114g/mol
Whole number multiple = 78.114g/mol
13.019g/mol
= 6
Molecular formula = C6H6