1.9 Percentage Composition

So far, we were dealing with the number of entities present in a given sample. But many a time, information regarding the percentage of a particular element present in a compound is required.

Suppose, an unknown or new compound is given to you, the first question you would ask is: what is its formula or what are its constituents and in what ratio are they present in the given compound?

For known compounds also, such information provides a check whether the given sample contains the same percentage of elements as present in a pure sample. In other words, one can check the purity of a given sample by analyzing this data.

Let us understand it by taking the example of water (H2O). Since water contains hydrogen and oxygen, the percentage composition of both these elements can be calculated as follows:

Mass % of an element = (mass of that element in the compound X 100) / (molar mass of the compound)

Molar mass of water = 18.02 g

Mass % of hydrogen = (2 X 1.008) X 100 / (18.02) =11.18

Mass % of Oxygen = (16.00) X 100 / (18.02) = 88.79

Let us take one more example. What is the percentage of carbon, hydrogen and oxygen in ethanol?

Molecular formula of ethanol is: C2H5OH Molar mass of ethanol is:

(2×12.01 + 6×1.008 + 16.00) g = 46.068 g

Mass per cent of carbon = (24.02 X 100) / 46.068 = 52.14 %

Mass per cent of hydrogen = (6.048g / 46.068g) X 100 = 13.13 %

Mass per cent of oxygen = (16.00 g / 46.068g) X 100 = 34.73 %

After understanding the calculation of per cent of mass, let us now see what information can be obtained from the per cent composition data.

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