CALCULATING THE EMPIRICAL FORMULA OF ORGANIC COMPOUNDS!!!

The empirical formula of an organic compound (any substance that contains carbon) can be found by:
-burning the compound (react with O2)
-collecting and weighing the products
-from the mass of the products, the moles of each element in the original organic compound can be calculated


Ex. 5g of sample→15.0g of CO2 & 8.18g of H2O
     
      CxHy+zO2→xCO2+y/2H2O
     
      this is the balanced chemical equation for the burning of CxHy
      
      the moles of C and H in CxHy=moles of C in CO2 and the moles of H in H2O
     
     15g*(1mole/44g)=0.341mole C      8.18g*(1mole/18.02g)=0.454mole water*2=0.908mole H
      
     0.908 moles of H and 0.341 moles of C must have been in the original substance
CxHy=C0.341H0.908
    
    Carbon: 0.341/0.341=1           Hydrogen=0.908/0.341=2.66*3=8
     
    Empirical Formula=C3H8
      
    *always check the masses

Empirical+Molecular Formula

Empirical formula definition: gives the lowest term ratio of atoms (or moles) in the formula

*All ionic compounds are empirical formulas

Reference from wiki:

http://en.wikipedia.org/wiki/Empirical_formula

Ex. C₄H₁₀ (butane) à molecular formula

àC₂H₅

Ex. consider that we have 10.87g of Fe and 4.66g of O, what is the empirical formula

#1 convert g à mole

Fe 10.87g×1mol/55.8g = 0.195mol

O  4.66g×1mol/16.0g = 0.291mol

#2 divide both by the smallest molar amount

Fe 1    O 1.5

#3 scale ratios to whole numbers

Fe 1×2=2

O 1.5×2=3

Therefore empirical formula = Fe₂O₃

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

To calculate multiple:

      Molar mass of the compound

N=--------------------------------------------------------------------

  Molar mass of the empirical formula

Reference from wiki:

http://en.wikipedia.org/wiki/Molecular_formula

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

MM C₂H₅=12×2+1×5=29g/mol

N=(58g/mol)/(29g/mol)=2

Mf=2(C₂H₅)=C4H10

Ex. 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 7.74×(1mol/12g)=0.62  à1

H 1.24×(1mol/29g)=1.24  à2

O 9.92×(1mol/16g)=0.62  à1

EF=CH₂O àMM=30g

N=180g/30g=6 C₆H₁₂O₆(Glucose)