Ca = Ca (only one Ca) , (1*2)N = 2N (two nitrogens) , (3*2)O = 6O (six oxygens)
It is not written Ca2O6 but
Ca(NO3)2 - why is that?
That is because there is more information in Ca(NO3)2 than Ca2O6. By the molecular formula we can see that Ca(NO3)2 contain two NO3 - groups. Other groups used in a typical molecular 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
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
common factor -->
Chemistry Software - Home