Percentage Composition

Introduction

The percentage by mass of each element present in a compound is defined as Percentage Composition. It is calculated from the molecular formula by dividing the mass of a single element in one mole of a compound by the mass of one mole of that compound. The value is defined or expressed as a percentage.

Example: The percentage composition of water is 20% hydrogen and 80% oxygen.

 2 atoms of Hydrogen are found in water, and one mole of water is 18.0152 grams. 1.008 grams is one mole of a hydrogen atom. Thereby 2.016 grams is the weight of two hydrogen atoms. Therefore, one mole of water has 2.016 grams of Hydrogen. Thus, the percentage composition of Hydrogen would be 2.016/18.0152 = 11.19%. 

Now let us study in detail Percentage composition and its formula.

The percentage composition of a compound is expressed in terms of all the elements it contains, which further helps in the chemical analysis of the given compound.

Percent Composition Formula

The percentage composition of a given element can be expressed by using the formula below:

% CE =gE / gT×100

where, % CE is the percentage composition of the element E. The numerator (gE) indicates how much of element E is present in the compound. Whereas the denominator (gT) is the total amount of all the elements present in that compound.

To get the percentage form of the composition this ratio is multiplied by 100

Mass Percentage Composition

It denotes the concentration of an element in a compound or a component in a mixture. This term signifies the total percent by mass of each element present in a compound.

To calculate, just divide the mass of the component by the total mass of the mixture. Further, multiply the ratio by 100. It is also termed as the mass percent(w/w) %. 

Importance of Mass Percentage

For many scientific and practical reasons, it’s important to know what quantity of an element is present in a given compound. This is indispensable for a chemist to get to the empirical formula of any compound.

The lowest whole number of moles as well as the relative number of atoms of each element in a compound can be understood through this formula. For chemists can also calculate the actual molecular formula by using the empirical formula. The formula helps in figuring out the number of atoms present in a compound.

Determination of the Mass Percent from a Chemical Formula

In this section, we will explain it with the help of an example to make you understand the fundamentals of how to calculate the mass percent. Let us consider methane. The chemical formula for methane is CH4. Let us calculate the mass percent of each of the elements in methane.

The formula shows the number of moles of each of the elements. There is 1 carbon atom and 4 hydrogen atoms in Methane.

By multiplying each of the atoms by its molar mass, we arrive at the mass of each of the elements in methane.

To reach the mass fraction of each element, we need to divide each element’s mass by the mass of 1 mole of methane. To get the mass percentage of elements, multiply the derived value by 100.

Calculation:

Step 1 – Determine the mass of 1 mole of CH4

The mass that each mole of Carbon has is 12.01g/mol of Carbon. This, we know, from the periodic table. One mole of Hydrogen has a mass of 1.008g/mol of Hydrogen. Therefore, 4 moles of Hydrogen will have a mass of 4 x 1.008 = 4.032g of Hydrogen.

Therefore, 1 mole of methane (CH4) has a total mass of 12.01 + 4.032= 16.042 g/mol

Step 2 – Determine the mass fraction of each element in 1 mole of methane

But before that, let us understand what is meant by Mass Fraction?

It is the mass that each element contributes or comprises to the total mass of Glucose. 

To get mass percentage of each element, we multiply this by 100

Mass fraction of Carbon = 12.01g/16.042g = 0.7486

Therefore, mass % of Carbon= 0.7486×100 = 74.86%

Mass fraction of Hydrogen = 4.032/16.042 = 0.2513

Therefore, mass % of Hydrogen= 0.2513 x 100 = 25.13%

It is interesting to note that even though Carbon has less number of moles of the compound compared to Hydrogen, its mass percentage is higher than Hydrogen. This is because Carbon has a higher molar mass than Hydrogen.

Solved Example 

Q1: Find the percentage composition of each element in Glucose.

Solution: The chemical formula for glucose is C6H12O6. We now calculate the mass percent of each of the elements in Glucose.

As seen from the chemical formula, Glucose has 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Now multiplying each of the atoms by its molar mass will give us the mass of each of the elements in Glucose.

We now divide each mass by the mass of 1 mole of Glucose to arrive at the mass fraction of each element. To get the mass percentage of the elements, multiply this value by 100. 

Calculations –

Each mole of Carbon has a mass of 12.01g/mol of Carbon, which we know, from the periodic table. Therefore, 6 moles of Carbon will have 12.01g/mol x 6 = 72.06g of Carbon.

1.008g/mol of Hydrogen is the mass of 1 mole of Hydrogen. Accordingly, 12 moles of Hydrogen have a mass of 12 x 1.008 = 12.096g of Hydrogen.

In the case of Oxygen, the mass in 1 mole of Oxygen is 16.00g/mol. For 6 moles of Oxygen, it is calculated as 16.00 x 6 = 96 g of Oxygen. 

Next, 1 mole of Glucose (C6H12O6) contains a total mass – 72.06 +12.096 + 96 = 180.16 g/mol

Determining the Mass Percentage – 

Carbon – Mass fraction = 72.06g/180.16g = 0.4000

Mass % of Carbon= 0.4000×100 = 40.00%

Hydrogen Mass Fraction = 12.096/180.16 = 0.06714

Mass % of Hydrogen = 0.06714 x 100 = 6.714%

Oxygen Mass fraction =96/180.16 = 0.53286

Mass% to Oxygen = 0.53286 x 100 = 53.286%

It is interesting to note that even if Carbon and Oxygen have an equal number of moles of the compound, their mass percentages are different. Oxygen has a molar mass higher than that of Carbon, thereby Oxygen has a higher value.

Carbon by Mole in methane – The percentage composition

As per the rule book of chemistry, a mole is stated as the amount of a substance that contains exactly 6.02214076 × 1023 ‘elementary entities’ of the given substance.

The number 6.02214076 × 1023 is popularly known as the Avogadro number and the value of the Avogadro constant is many a time denoted by the symbol ‘NA’

Atoms, monatomic, molecules, or polyatomic ions, and other particles (such as electrons) are the elementary entities that can be represented in moles.

• 16 g/mol is the molecular mass in one mole of methane

• CH4 is the chemical formula for Methane is a hydrocarbon 

• There is one atom of carbon and four atoms of Hydrogen in Methane. 

• It is a hydride of group-14 and the simplest alkane and is the main constituent of natural gas.

In one mole of methane, there is one mole of carbon atoms and 4 moles of the hydrogen atom, so the percentage composition of C by mole in methane is,

The % for C atom in methane is = 12/16×100, which makes percentage for C = 75%

That’s why one mole of methane has 75 % of Carbon.

Conclusion

Percentage composition of a compound is defined as the ratio of the amount of each element to the amount of individual elements in the compound and multiplied by 100. The representation is a composition of all elements in the very compound. 

Formula: Percentage composition= Mass or amount of element present/ Total mass or amount of compound *100

The significance of the calculation of the composition is found in the chemical analysis.