Education Journalist | Study Abroad Lead
The percent composition of a particular compound is the number of individual elements in the compound multiplied by 100. It is used to understand the contribution made by a particular element in a given chemical compound. It is used in stoichiometric calculations where the percentage contributed by an element is important. The percentage of each element is calculated in terms of mass (SI unit gram). In this article, we will have a look at the percent composition formula and look at some solved examples related to the same.
| Table of Contents |
Key terms: Percent composition, percent composition formula, element, chemical compound, molecular formula.
What is percentage composition?
[Click Here for Sample Questions]
The percent composition is nothing but the ratio of the amount of each element present in the compound to the total amount of compound multiplied by 100. The percent composition is measured in grams of the element present in the given chemical compound.
The percentage composition of any chemical compound is an expression of its composition in terms of all the elements present. It is mostly used in the chemical analysis of the given compound.
Also Read: Mass percent formula
Percent composition formula
[Click Here for Sample Questions]
The percent composition formula of an element in a particular compound is given by –
% composition = \(\frac{grams of element}{grams of compound} \times 100\)
It is also expressed mathematically as %CE = (gE/gT) x 100.
Where, %CE stands for percentage composition of the element to be calculated,
gE stands for grams of the element, and
gT stands for the total mass of the compound in grams.
Also Read: Number of moles formula
Properties of percent composition formula
[Click Here for Sample Questions]
The formula is based on the mass percent composition of a compound.
- The percent composition is constant for a particular given chemical compound, no matter what amount of the compound is used.
- It is basically the ratio of each element. When multiplied by 100 it gives the percentage of that particular element in the given compound.
- The percent composition formula can also be used to obtain the molecular formula of any chemical compound. When a new compound is discovered, one can get the ratio of each element from its molecular mass and percent composition formula, and then can use it to determine the molecular formula of the compound.
Also Read: Difference between an element and chemical compound
Things to remember
- Percent composition of a given chemical compound is given by the total grams of the particular element in the total amount of the compound multiplied by 100. It gives the percentage of mass contributed by one particular element in the compound.
- The formula to calculate the percent composition of any given compound is
%CE = (gE/gT) x 100
Where, %CE is the percentage composition of the element to be calculated,
gE is grams of the element, and
gT is the total mass of the compound in grams.
- Percent composition of a given compound is fixed, it does not change with the amount of the compound.
- The percent composition formula is used often in chemical analysis.
- It is also used to determine the molecular formula of a newly discovered compound.
Solved examples
Ques 1. Calculate the percent composition of calcium and oxygen in the calcium hydroxide compound. [4 marks]
Answer: The chemical formula of calcium hydroxide is Ca(OH)2
To calculate the percent composition, we need the molar mass of Calcium and oxygen.
Molar mass of Calcium, Ca = 40 g/mol.
Molar mass of Oxygen, O = 16 g/mol.
Molar mass of Hydrogen, H = 1 g/mol.
Thus, the total molar mass of the compound = mass of Ca + 2*(mass of O) + 2*(mass of H)
= 40 + 2*16 + 2*1 = 40 + 32 + 2
= 74 g/mol.
So the percentage composition by mass of calcium in calcium hydroxide is,
%Ca = (40/74)*100 = 54.05%.
Similarly, the percentage by mass of oxygen is,
%O = (16/74)*100 = 21.62%.
Ques 2. Calculate the percentage by mass of each of the elements of water. [4 marks]
Answer: The molecular formula of water is H2O (Dihydrogen oxide)
To calculate the percent composition of a water molecule, we need the molar mass of each element in the water molecule.
Molar mass of Hydrogen, H = 1 g/mol.
Molar mass of Oxygen, O = 16 g/mol.
Therefore, The total molecular weight of water = 2*(mass of H) + mass of O.
= 2*1 + 16 = 2 + 16 = 18 g/mol.
Hence, the percent composition by mass of Hydrogen in water is,
%H = (1/18)*100 = 11.11%.
Similarly, the percent composition by mass of oxygen is given as,
%O = (16/18)*100 = 88.89%.
Ques 3. Calculate the mass percent of each of the elements in glucose. [5 marks]
Answer. The molecular formula of glucose is C6H12O6. It contains 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen.
The molar of each element are given below:
Molar mass of C = 12g/mol
Molar mass of H = 1g/mol
Molar mass of O = 16g/mol
Therefore, the molar mass of glucose is 6*12 + 12*1 + 6*16
= 180g/mol
So, the percent composition of each element in glucose is
%C = (72/180)*100 = 40%
%H = (12/180)*100 = 6.67%
%O = (96/180)*100 = 53.3%
Ques 4. What is the percentage of carbon in carbon dioxide (CO2)? [2 marks]
Answer. The molecular formula or carbon dioxide is CO2.
The molar mass of each element of CO2 are:
Molar mass of carbon = 12 g/mol
Molar mass of oxygen = 16g/mol
Therefore the molar mass of CO2 is 12 + 16*2 = 44g/mol
Hence the percentage of carbon in CO2 is
12/44 x 100 = 27.27%.
Ques 5. What is the percentage of carbon, hydrogen and oxygen in ethanol (C2H5OH)? [3 marks]
Ans. Molecular formula of ethanol is : C2H5OH
Molar mass of ethanol is : (2×12.01 + 6×1.008 + 16.00) g = 46.068 g
Hence, Mass percent of carbon = 24.02g ×100 / 46.068g = 52.14%
Mass percent of hydrogen = 6.048g ×100 / 46.068g = 13.13%
Mass percent of oxygen = 16.00 g ×100 / 46.068g = 34.73%
Ques 6. A compound contains 4.07 % hydrogen, 24.27 % carbon and 71.65 % chlorine. Its molar mass is 98.96 g. What are its empirical and molecular formulas? [5 marks]
Answer. Since the percent composition of the compound is given, to determine the empirical formula of the compound we need the number of moles of each element present in the compound.
As we know, no. of moles = given mass/molar mass
Therefore, no. of moles of H = 4.07 / 1 = 4.07 moles
No. of moles of C = 24.27 / 12 = 2.021 moles
No. of moles of Cl = 71.65 / 35.5 = 2.021 moles
Therefore, the ratio of each element to other is 2:1:1 for H:C:Cl
So the empirical formula of the compound is CH2Cl
Now, to determine the molecular formula from the empirical formula, we need the molar mass of the compound, given as 98.96g/mol
For CH2Cl, empirical formula mass is 12.01 + 2 × 1.008 + 35.453 = 49.48 g
Molar mass / Empirical formula mass = 98.96 / 49.48g = 2
Multiply empirical formula by n obtained above to get the molecular formula
Hence the molecular formula is C2H4Cl2 .
Ques 7. Find out the percent composition of Cl in Hydrochloric acid (HCl). [2 marks]
Answer. Molar mass of H is 1g/mol
Molar mass of Cl is 35.5 g/mol
Therefore, the molar mass of HCl is 35.5 +1 = 36.5 g/mol
So the percent percent formula of Cl in HCl is (35.5 / 36.5) x 100
%Cl = 97.26%
Ques 8. A compound has the following percentage composition: 40.0% carbon, 6.72% hydrogen, 53.28% oxygen. How many moles of hydrogen are present in 100 g of the compound described above? [1 marks]
a) 3.33 moles
b) 12 moles
c) 6.72 moles
d) None of the above
Answer. (c) 6.72 moles
Explanation. In 100 grams of the compound, there is 6.72g of hydrogen. The molar mass of Hydrogen is 1g/mol
Therefore, the number of moles of hydrogen in 100g of compound is
6.72/1 = 6.72 moles
Ques 9. What is the mass percent of sodium in sodium chloride (NaCl)? [1 marks]
a) 50%
b) 66.66%
c) 33.5%
d) 39.3%
Answer. (d) 39.3%
Explanation. To calculate the mass percent of Na in NaCl we need the molar mass of both Na and Cl.
Molar mass of Na = 23 g/mol
Molar mass of Cl = 35.5 g/mol
Hence, the molar mass of NaCl = 23 + 35.5 = 58.5 g/mol
Therefore, the mass percent of Na in NaCl is (23/58.5) x 100
% Na = 39.3%
Ques 10. Determine the percent composition of sulphur in sulphuric acid (H2SO4). [4 marks]
Answer. The chemical formula of calcium hydroxide is H2SO4
To calculate the percent composition, we need the molar mass of hydrogen, sulphur, and oxygen.
Molar mass of hydrogen, H = 1 g/mol.
Molar mass of Sulphur, S = 32 g/mol.
Molar mass of Oxygen, O = 16 g/mol.
Thus, the total molar mass of the compound = 2*mass of H + mass of S + 4*(mass of O)
= 2x1 + 32 + 4x16
= 98 g/mol.
So the percentage composition by mass of Sulphur in sulphuric acid (H2SO4) is,
%S = (32/98)*100 = 32.65%.
Read Also:
Comments