NCERT Solutions for Class 11 Chemistry Mass Percentage Composition
In this article, we discuss the topic ‘Mass Percentage Composition of an element in a given compound’. This covers 11 science, the Unit 1 of Chemistry of Class 11.
MASS PERCENTAGE COMPOSITION
The mass percentage is one way of representing the concentration of an element in a compound or a component in a mixture. It is used to describe the percent by mass of each element in a compound.
The mass percentage is calculated as the mass of a component divided by the total mass of the mixture, multiplied by 100. It is also known as mass percent(w/w) %.
Importance of mass percentage
For many purposes, it is important to know how much of an element is present in a given amount of a compound. It helps the chemists to derive an empirical formula ( It shows the lowest whole number of moles, and thus the relative number of atoms of each element in a compound)of the given compound, and the empirical formula can help in finding out the actual molecular formula(it shows the actual number of atoms of each element in a molecule).
Determining the Mass Percent from a chemical formula
Example: To calculate mass percentage of each element in glucose (C6H12O6)
Solution: We know the relative amounts (mol) of the elements from the formula, 6Carbon atoms, 12 Hydrogen atoms and 6 Oxygen atoms. Now we multiply the amount of each atom in glucose by its molar mass to find the mass of each element of glucose. Divide each mass by mass of 1 moles of glucose gives the mass fraction of each element, and then multiplying by 100 gives a mass percentage of the elements.
From the chemical formula, we know that there are 6 moles of carbon in 1 mole of glucose.
1 mole of carbon has a mass of 12.01g/mol of carbon (from periodic table)
So 6 moles of carbon will have 12.01g/mol x 6 = 72.06g of Carbon.
Similarly, 1mole of Hydrogen has a mass of 1.008g/mol of Hydrogen (from periodic table)
So, 12 moles of Hydrogen will have the mass of 12 x 1.008 = 12.096g of Hydrogen.
In the same way, for Oxygen, 1 mole of oxygen has a mass of 16.00g/mol(from periodic table)
So, 6 moles of oxygen will have 16.00 x 6 = 96 g of Oxygen.
Thus, 1 mole of Glucose (C6H12O6) has a mass of 72.06 +12.096 + 96 =180.16 g/mol
To find out the mass % of the three elements of glucose, we first calculate the mass fraction of each element in 1 mole of glucose. Mass fraction is the mass contributed by each element in the total mass of glucose. Mass fraction multiplied by 100 gives a mass percentage of each element.
Mass fraction of Carbon= 72.06g/180.16g =0.4000
Mass % of Carbon= 0.4000×100= 40.00%
Mass fraction of Hydrogen = 12.096/180.16 =0.06714
Mass% of Hydrogen= 0.06714 x 100=6.714%
Mass fraction of Oxygen =96/180.16 =0.53286
Mass% of Oxygen = 0.53286 x 100 = 53.286%
Though there is an equal number of moles of Carbon and Oxygen, in the compound, the mass % of Oxygen is greater than that of Carbon because the molar mass of Oxygen is greater than that of Carbon. The mass % of Hydrogen is small because the molar mass of H is small. But we see that the sum of mass % of all the elements in a compound is 100%.
In the given example, 40.00% + 6.714% + 53.286% = 100.00%
Determining the Mass of an Element from its Mass Percent
The above example shows that an element always constitutes the same fraction of the mass of a given compound. The mass fraction of an element in a given compound thus obtained remains constant and can be used to find out the mass of the element in any mass of that compound.
Example: To find out the mass of Carbon in 16.55 grams of Glucose (C6H12O6)
Solution: We know that 1 mole of glucose has a mass of 180.16 grams
In 1 mole of glucose, there are 6 atoms of Carbon, which contributes the mass fraction of 72.06/180.16 = 0.4000
Thus the mass percent is equal to 0.4000 x 100 =40%
This means in 1 mole or 180.16 grams of glucose, 40% is Carbon element and it remains same, whatever may be the quantity of glucose.
Therefore the mass of Carbon in 16.55 grams of Glucose will also be 40%
Mass of carbon in grams= (72.06/180.16) x 16.55
=6.62 grams
In other words, 40% of 16.55 is 6.62 grams.
Thus we now know how to calculate the Mass Percentage of an element in a given compound (knowing the chemical formula of the given compound), and vice versa i.e. to find out the chemical formula (Empirical / Molecular Formula) of a compound by knowing the masses of the element in a compound.
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In 11th Science you can visit another notes, NCERT Solutions for Class 11 Chemistry
Determining Empirical and Molecular Formula of Compounds
Chemistry Class 11 : Molar Mass
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