Dec 3--Empirical + Molecular Formula

Empirical Formula:

gives the lowest term ratio of atoms (or moles) in the formula. (All ionic compounds are empirical formulas.)

Ex. C4H10—molecular formula     C2H5—empirical formula
 

Example#1

Consider that we have 10.87g of Fe and 4.66g of O, what is the empirical formula?

1.Convert g into moles                              
 Fe: 10.87g of Fe* （1 mole/55.8g）=0.195 moles
 O: 4.66g of O * (1 mole/16g) = 0.291 moles

2.Divide both of them by the smallest molar amount
Fe: 0.195/0.195=1       
O: 0.291/0.195 = 1.5

3.The last step is to scale ratios to whole numbers
Fe: 1 * 2 = 2        
O: 1.5* 2 = 3
 

Ans:Fe2O3

Example#2

A compound contains 31.9% k, 28.9%Cl, and 39.2% O. What is the empirical formula?
 
K: 31.9g * (1 mole/ 39.1g) = 0.816 mol
Cl: 28.9g * (1 mole/35.5g) = 0.814 mol
O: 39.2g * (1 mole/16g) = 2.45 mol
Repeat the second and third step.

Molecular Formula: 

is a multiple of the empirical formula and shows the actual number of atoms that combine to form a molecule.
 
n= (molar mass of the compound/molar mass of the empirical formula) 

Example#1

A molecule has an empirical formula of C2H5 and a molar mass of 58g/mol what is the molecular formula?

MM C2H5 = 29g/mol
n = (58g/mol / 29g/mol) = 2
MF = 2(C2H5) = C4H10 = BUTANE

Example#2

The empirical formula of a gas is CH2 what is the molecular formula if the molar mass  is 42g/mol

MM of CH2 = 14g/mol
n= (42g/mol / 14g/mol) = 3
MF = 3(CH2) = C3H6

Example#3

 A compound contains 7.44g C, 1.24g H, and 9.92g O , the molar mass of the compound is 180g what is the molecular formula?

C 7.44g * (1 mole/ 12.0g) = 0.62mole     1
H 1.24g * (1mole/ 1.0g) =1.24 moles      2
O 9.92g * (1 mole/ 16.0g) = 0.62 mole    1
CH2O = 12.0+1.0*2 +16.0=30g/mol
n = (180g/mol / 30g/mol) = 6
MF = 6(CH2O) = C6H12O6