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.

In chemistry, understanding how much of each element is present in a compound can tell us a lot about that compound. This concept is referred to as 'percentage composition.' It helps us determine the makeup of substances and assess their purity, which can be crucial in various applications, whether in laboratories or industries.

Think of a cake recipe. If you were to bake a cake made of flour, sugar, and eggs, knowing the percentage of flour, sugar, and eggs will help you understand how the cake will taste and its texture. Similarly, knowing the percentages of elements in a compound helps chemists predict its behavior and applications.

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

To find the percentage of an element in a compound, you use the formula:

1. First, determine the mass of the element in the compound.
2. Then, divide this mass by the total molar mass of the compound.
3. Finally, multiply by 100 to convert the value into a percentage. This gives us the mass percentage of that element within the compound.

Consider a bottle of soda that contains water, sugar, and carbon dioxide. If you want to find out how sweet your soda is, you could calculate the percentage of sugar it contains relative to the total mass of the soda. The same concept applies here when we determine the percentage of an element in a 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 analysing this data.

Using water as an example, its molar mass is about 18.02 grams per mole. The mass percentage of hydrogen and oxygen can be calculated as follows:

• For hydrogen: Mass % of hydrogen = (2.016 g / 18.02 g) × 100 = 11.18%.
• For oxygen: Mass % of oxygen = (16.00 g / 18.02 g) × 100 = 88.79%. This shows that water is primarily composed of oxygen, with hydrogen making up a smaller percentage.

If you have a bottle of pure water and another bottle that contains some impurities, by calculating the percentage composition of water (the percentage of H2O), you can assess how pure your sample is compared to what is expected. This is similar to tasting a dish that should have a certain flavor; if it tastes off, you can check the ingredients and their proportions.

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.

To find the percentage composition in ethanol, you start by calculating the molar mass, which is 46.068 g. Then, determine the mass percentages of each element:

• Mass of carbon in ethanol = (2 × 12.01 g = 24.02 g),
• Mass of hydrogen = (6 × 1.008 g = 6.048 g),
• Mass of oxygen = 16.00 g. The mass percentages would then be calculated as follows:
• % of Carbon = (24.02 g / 46.068 g) × 100 = 52.14%
• % of Hydrogen = (6.048 g / 46.068 g) × 100 = 13.13%
• % of Oxygen = (16.00 g / 46.068 g) × 100 = 34.73%.

When you analyze your favorite drink like a fruit juice or soda, knowing the percentage of sugar, water, and other ingredients helps you understand the taste. For ethanol, knowing the percentages of carbon, hydrogen, and oxygen can reveal insights into its properties and applications, similar to how you might want to know how much sugar is in your beverage.

An empirical formula represents the simplest whole number ratio of various atoms present in a compound, whereas the molecular formula shows the exact number of different types of atoms present in a molecule of a compound.

The empirical formula simplifies a compound to its most basic ratio. For instance, for glucose (C6H12O6), the empirical formula is CH2O, which shows the simplest ratio of carbon, hydrogen, and oxygen atoms. In contrast, the molecular formula provides the exact count, which in this case reflects the actual number of carbon, hydrogen, and oxygen atoms in each molecule of glucose.

Think of a recipe. The molecular formula is like a detailed recipe that tells you exactly how many of each ingredient you need, while the empirical formula is like saying you only need 'two parts of sugar for every one part of flour.' Both serve different purposes, much like how understanding both formulas helps in chemical analysis.

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