Question

How do you find an empirical formula from mass percent?

Answer 1

Hint:‌The empirical formula of a molecule is a formula which gives an idea about the lowest ratio of the constituent elements in whole numbers. It does not provide any idea about the spatial arrangement of the constituent atoms, which are present in the molecule.


Complete step by step answer:
The procedure to find out an empirical formula by using its mass percentage can be divided into six simple steps. It is easier to understand the method in a tabular form, which we would make at the end of the steps, as an example.
In the first step, we would assign the symbol of each atom, by which it is represented. For example $N$ for nitrogen, or $O$ for oxygen.
In the next step, we would use the mass percentage of every element present in the compound which would be provided to us in the question itself. For example, $C-48%$, $O-15%$.
Now, we would write the atomic masses of every element, such as the carbon would have atomic mass $12$.
In the next step we would divide these values of mass percentage with their respective atomic mass values of these elements, so as to get the relative number of moles, of each of them.
Next, we will divide these ratios, we got from above step, of all the elements by the ratio which has the smallest relative number of moles.
Now, we are supposed to get a whole number ratio, and in case we don’t, then we would just multiply all of the numbers using a specific whole number and then we will write the ratio of the whole number of the respective elements.
Now, we are going to take an example to understand more clearly.
Suppose the mass percentage of a compound after analysis gave the following composition \[Na=14.31%{ }\] \[S=9.97%{ }\] \[H=6.22%{ }\] \[O=69.5%\]. We will calculate the empirical formula of this compound using the following data. Molecular mass of the compound is \[322\]. \[Na=23{ }\] \[S=32{ }\] \[O=16{ }\] \[H=1\].

So, the empirical formula becomes, $N{{a}_{2}}S{{O}_{14}}{{H}_{20}}$.

Note:Empirical Formula of a compound is the expression which gives an idea about the actual molecular formula of a compound by taking the simplest subscripts of the constituent atoms of that molecule. It can be derived by using the mass percentage of the constituent atoms which are present in the considered molecule along with their atomic masses, in the form of a table.