Percentage Composition – HSC Chemistry


This is part of the HSC Chemistry course under the topic of Properties of Matter.

HSC Chemistry Syllabus

  • Explore homogeneous mixtures and heterogeneous mixtures through practical investigations:

– Calculating percentage composition by weight of component elements and/or compounds (ACSCH007)

    Percentage Composition

    This video will introduce the idea of percentage composition and demonstrate how to calculate percentage composition given mass and vice versa


    What is Percentage Composition?

    Compounds are substances consisting of two or more different elements chemically bonded together. The composition of a compound can be described in terms of the percentage by mass of each element in a given sample of the compound. Understanding the percentage composition by mass of each element within a compound is fundamental in various scientific and industrial processes.

    In the industrial sector, especially in metal extraction, knowing the percentage composition of elements in minerals is crucial. For instance, in the extraction of iron from iron ore (composed mainly of iron(III) oxide, `Fe_2O_3`), the percentage compositions helps in determining the yield of pure iron.


    Calculating Percentage Composition Given Mass

    Consider the following question: 

    "Calculate the percentage composition of iron in a 100 g sample of iron(III) oxide, given it contains 70 g of iron." 

    Since iron(III) oxide is composed only of iron and oxygen, given that 70 g of it is iron, the rest must be oxygen. To find the percentage composition of a particular element in the compound, divide the mass of the element by the total mass and multiply the value by 100. 


    $$\text{Percentage of iron }= \frac{\text{mass of iron}}{\text{total mass of compound}} \times 100 =\frac{\text{70 g}}{\text{100 g}} \times 100 = \text{70%} $$


    Example 1: Calcium Carbonate (`CaCO_3`) (Example analysed in video)

    Calculate the percentage composition of calcium, carbon, and oxygen in an 88g sample of calcium carbonate, given the masses of calcium, carbon, and oxygen are 35 g, 11 g, and 42 g respectively. 


    $$\text{Percentage of Calcium }= \frac{\text{35 g}}{\text{88 g}} \times 100 = \text{39.8%}$$

    $$\text{Percentage of Carbon }= \frac{\text{11 g}}{\text{88 g}} \times 100 = \text{12.5%}$$

    $$\text{Percentage of Oxygen }= \frac{\text{42 g}}{\text{88 g}} \times 100 = \text{47.7%}$$


    Calculating Mass Given Percentage Composition

    Just as we can calculate the percentage composition of a compound given its mass, we can also calculate the mass of each constituent element if we know the percentage composition of the compound by multiplying the total mass by the mass percentage. 

    Example 2: Iron(II) Sulfate (`Fe_2SO_4`) 

    A 152 g sample of Iron(II) Sulfate contains 27% iron, 21% sulphur, and the rest is oxygen. Determine the mass of each element. 


    $$\text{Mass of iron} = \frac{152 \times 37}{100} = \text{56.24 g ~ approx 56 g}$$

    $$\text{Mass of Sulfur} = \frac{152 \times 21}{100} = \text{31.92 g ~ approx 32 g}$$

    $$\text{Mass of Oxygen} = \text{Total mass – Mass of Iron – Mass of Sulfur}  = \text{152 g – 56 g – 32 g = 64 g} $$



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