Empirical Formula and Molecular Formula

The chemical formula of a compound can be determined to establish the proportions by mass of the constituent elements that are contained in the compound. In case the elements of a compound are known, it is possible to use the respective RAM (relative atomic mass)  to compute the ratio in which they reacted to yield final products. The chemical formulae are divided into empirical and molecular formulae as explained below.

 

Empirical Formula

The empirical formula of compounds represents the proportion in which atoms can combine to form a compound. While the molecular formula, on the other hand, expresses the exact number of different types of atoms present in the molecule of a compound. If a compound's molecular formula cannot be reduced any more, then the empirical formula is the same as the molecular formula. Below is an example of calculating the empirical formula.

 

  

 

Steps for Calculating the Empirical Formula:

  1. Start with the number of grams of each element, given in the problem. If the percentages are mentioned instead of grams then, assume that the total mass is 100 grams so that the mass of each element is equal to the percentage given.

  2. Then convert the mass of each element to moles using the molar mass from the periodic table.

  3. Divide each mole value by the smallest number of moles calculated.  Round to the nearest whole number and this is the mole ratio of the elements and is represented by subscripts in the empirical formula. If the number is too far to round (x.1 ~ x.9), then multiply each solution by the same factor to get the lowest whole number multiple. For example, if one solution is 1.5, then multiply each solution in the problem by 2 to get 3.

  4. Once the empirical formula is found, the molecular formula for a compound can be determined if the molar mass of the compound is known.  Simply calculate the mass of the empirical formula and divide the molar mass of the compound by the mass of the empirical formula to find the ratio between the molecular formula and the empirical formula. 

  5. Multiply all the atoms (subscripts) by this ratio to find the molecular formula. So, Molecular formula= (Mass of empirical formula) n

 

A compound of carbon, hydrogen, and oxygen contains 54.55% carbon, 9.09% hydrogen, and 36.6% oxygen. If its relative molecular mass is 88, what is its molecular formula?

RAM=(C=12, O=16, H= 1).

 



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