Molecular Formula


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Molecular Formula - represent the actual numbers of atoms of the different elements in one molecule of a compound. An example is Ca(NO3)2

Ca = Ca (only one Ca) , (1*2)N = 2N (two nitrogens) , (3*2)O = 6O (six oxygens)

It is not written CaN2O6  but Ca(NO3)2  - why is that?
That is because there is more information in Ca(NO3)2 than CaN2O6. By the molecular formula we can see that Ca(NO3)2 contain two NO3 - groups. Other groups used in a typical moelcular formula: NO2 (nitrite) , SO4 (sulfate) , SO3  (sulfite) , OH (hydroxyl)...

Determination of the molecular formula by the empirical formula and mass percent composition

Example: A combustion analysis gives the following mass %:  H= 9.15%    C = 54.53%   O = 36.32% . Determine the molecular formula knowing that : molecular mass = 132.16 , empirical formula = C2H4O    

Solution

Assume a 100 gram sample which will convert the given percentages to gram amounts

(9.15 gram H)/(1 gram/mole) = 9.15 moles  

(54.53 gram C)/(12 gram/mole) = 4.54 moles

(36.32 gram O)/(16 gram/mole) = 2.27 moles  

Determination of the simplest moles ratio:

Divide each of the moles figures by the lowest of the three (in this case 2.27).

2.27 moles O /2.27 =1     ,    9.15 moles H/2.27 = 4      ,     4.54 moles C/2.27 = 2

2*12.0 + 4*1.0 + 1*16.0 = 44

Calculating the common factor:

The common factor defines the ratio (molecular formula)/(empirical formula):

132.16/ 44  =  3

Determination of the molecular formula :

The solution: Multiply C2H4O  by the common factor --> C(2*3)H(4*3)O(1*3)  =  C6H12O3

More about Empirical Formula
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