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Determining Chemical Formulae

The information regarding the type and number of atoms present in the molecule of a compound can be obtained experimentally. Two types of experiments can be carried out, namely, (1) to synthesize the compound from the requisite elements, and (2) to break the compounds into the simple fragments whose compositions are already known. The second method is commonly followed, especially in the case of organic compounds containing carbon and hydrogen. 

The compound is burnt in a closed vessel with excess of oxygen so that the carbon is converted into carbon dioxide and the hydrogen into water. By weighing the masses of carbon dioxide and water formed, the masses of carbon and hydrogen in the original compound can be determined. Usually, these data are provided in mass percent. The oxygen atoms already present in the compound are normally not stated and can be obtained by subtracting the sum of percentages of all other elements from 100.

The first step in determining chemical formula of a compound is to find out its empirical formula from the provided data on mass percentages. This is achieved by dividing mass percentages by the corresponding atomic masses. From the values so obtained, one can find out the simplest ratio of amounts of different elements present in the compound. Dividing or multiplying the whole ratio by a suitable number can remove any fraction involved in this ratio. From this ratio, the empirical formula of the compound can be written down by taking the corresponding number of atoms of each element.

The second step involves the determination of the exact number of atoms present in the compound. This is achieved by dividing the molecular mass by the empirical mass. The latter can be obtained from the empirical formula of the compound. The empirical formula is multiplied by the number obtained during the division of molecular mass by empirical mass. The resultant expression is the required molecular formula of the compound.
If the element composition of butyric acid is found to be 54.2 mass % C, 9.2 mass % H and 36.6 mass % O, determine its empirical formula. The molecular mass of butyric acid was found to be 88 amu. What is its molecular formula?

Step I To find the ratio of amounts of atoms from the given mass percentages.


Mass %

Mass taken

Amount taken



54.2 g

54.2 g/12 g mol-1 =4.5 mol



9.2 g

9.2 g/1 g mol-1 =9.2 mol



36.6 g

36.6 g/16 g mol-1 =2.3 mol

Thus, the required ratio of amounts is C : H : O , 4.5 : 9.2 : 2.3.

Step II   To find the simple ratio involving integers only. Dividing the above ratio by 2.3, we get
                     C: H: O: , 2 : 4 : 1

Hence, the empirical formula is C2H4O
Now, the empirical mass = (2 × 12 + 4 × 1 + 1 × 16) amu
                                    =44 amu

The given molecular mass = 88 amu

Therefore, the molecule is C4H8O2.

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