Percentage Composition Class 11 Notes | EduRev

Class 11 : Percentage Composition Class 11 Notes | EduRev

The document Percentage Composition Class 11 Notes | EduRev is a part of Class 11 category.
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What is Percentage Composition?
The percentage composition of a given compound is nothing but the ratio of the amount of each element to the total amount of individual elements present in the compound. We then multiply this value by 100. Here, we measure the quantity in terms of grams of the elements present in the solution.
The percent composition of any compound is an expression of its composition in terms of all the elements present. The significance of this composition calculation is found in the chemical analysis.
Percentage Composition Class 11 Notes | EduRev

The Formula for Percentage Composition 
We can express the percentage composition of a given element using the formula below:
Percentage Composition Class 11 Notes | EduRev
Here, % CE is the percentage composition of the element E. This is the value that we are going to calculate. The numerator on the right side indicates the total amount of element E present in the compound. On the other hand, the denominator is an expression for the total amount of all the elements present in the compound.

We multiply this ratio by 100 to get the percentage form of the composition.

Let us now look at the mass percentage of composition in more details. We will also look at its importance.

Mass Percentage Composition 
We use the concept of mass percentage composition to denote the concentration of an element in a compound or a component in a mixture. We use this term to signify the total percent by mass of each element that is present in a compound.

It is important to note that we can calculate the mass percentage composition by dividing the mass of a component by the total mass of the mixture. This ratio is then to be multiplied by 100. We also call it the mass percent(w/w) %.

Importance of Mass Percentage
For many scientific and practical reasons, we ought to know what quantity of an element is present in a given compound. This is indispensable for the chemists to get to the empirical formula of many compounds.
This formula helps in showing the lowest whole number of moles and the relative number of atoms of each element in a compound. With the help of the empirical formula, chemists can also calculate the actual molecular formula. This formula shows the exact number of atoms in the compound.

Determining the Mass Percent from a Chemical Formula
We will explain this section with the help of an example. It will clear your fundamentals on how to calculate the mass percent.
Let us consider glucose. The formula for glucose is C6H12O6.

Let us calculate the mass percent of each of the elements in glucose.

From the formula, we can see the number of moles of each of the elements. Glucose has 6 carbon atoms, 12 hydrogen atoms and also 6 oxygen atoms.

Let us now multiply each of the atoms by its molar mass. Thus, we arrive at the mass of each of the element in glucose.

We now divide each mass by mass of 1 moles of glucose to arrive at the mass fraction of each element. Multiplying this value by 100 gives the mass percentage of the elements.

The Example in Detail
Each mole of carbon has a mass of 12.01g / mol of carbon.

This, we know, from the periodic table. So, 6 moles of carbon will have 12.01g / mol x 6 = 72.06g of Carbon.

Similarly, 1 mole of Hydrogen has a mass of 1.008g / mol of Hydrogen.

Therefore, 12 moles of Hydrogen will have the mass of 12 x 1.008 = 12.096g of Hydrogen.

Going by the same logic for Oxygen, 1 mole of oxygen has a mass of 16.00g / mol.

Therefore, 6 moles of oxygen will have 16.00 x 6 = 96 g of Oxygen.

Thus, 1 mole of Glucose (C6H12O6) has a total mass of 72.06 + 12.096 + 96 = 180.16 g / mol.

Calculating the Mass Percentage 
To find out the mass % of the three elements of glucose, let us first calculate the mass fraction of each element in 1 mole of glucose. So, what do we mean by the mass fraction? It is the mass that each element contributes to the total mass of glucose. By multiplying this by 100, we get the mass percentage of each element.

Mass fraction of Carbon = 72.06g / 180.16g = 0.4000
Therefore, mass % of Carbon= 0.4000 * 100 = 40.00%
Mass fraction of Hydrogen = 12.096 / 180.16 = 0.06714
Therefore, mass % of Hydrogen= 0.06714 * 100 = 6.714%
Mass fraction of Oxygen =96 / 180.16 = 0.53286
Therefore, mass% of Oxygen = 0.53286 * 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 higher value in this as its molar mass is higher than that of carbon.

Solved Example 
Q. Find the percent composition of each element in water.

Solution. We know that the chemical formula for water is H2O.
Let us now calculate the molar mass of water. The molar mass of Oxygen = 16.00 * 1 = 16 g / mole and of Hydrogen = 1.01 * 2 = 2.02 g / mole
Now, using the molar mass of each of the given elements, we find out the percentage composition of each element in H2O. It is given as the ratio of the grams of the element to the grams of the total element in the compound, multiplied by 100. Calculating the percentage composition of Hydrogen,
% H = 2.02 / 18.02 * 100

Therefore, % H= 11.21 %
Calculating the percentage composition of Oxygen,
Therefore, % O = 16 / 18.02 * 100 = 88.79 %

Try yourself!
Q.1. The percentage of oxygen in is NaOH 
(a) 40 
(b) 60 
(c) 8 
(d) 10
Ans. (a)
Solution. Molar mass of NaOH = 23 + 16 + 1 = 40
% composition of oxygen = Percentage Composition Class 11 Notes | EduRev= 40%

Q.2. The percentage of nitrogen in urea is about
(a) 46
(b) 85
(c) 18
(d)48

Ans. (a)
Solution.
urea =  NH2-CO-NH2
total mass = 60
mass of nitrogen = 14,
mass of 2 molecules hence
m = 28
N% = mass of nitrogen / mass of urea x 100
= 28 / 60 x 100
N% = 46.6%

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