Lesson 6, Topic 7
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# Percentage Composition

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The percentage composition of each element in a compound can be determined by

1. Calculating the relative molecular mass.

2. Calculating the amount, in percentage, of each element in the compound.

Example I

Calculate the percentage by mass of Nitrogen in trioxonitrate (v) acid. (H = 1, N = 14, O = 16)

Solution:

HNO3

Â 1 + 14 + 16 x 3.

Relative Molecular Mass = 63, N = 14

% by mass of Nitrogen = $$\frac{14}{63} \: \times \: \frac{100}{1} \\ = \scriptsize 22.2 \%$$

Example II

Calculate the percentage composition of all the components elements in calcium trioxocarbonate (IV) (CaCO3)
(Ca = 40, C = 12, O = 16)

Solution:

Relative molecular mass = 40 + 12 + (16 x 3)Â  =Â  100

% by mass of calcium = $$\frac{40}{100} \: \times \: \frac{100}{1} \\ = \scriptsize 40 \%$$

% by mass of carbon = $$\frac{12}{100} \: \times \: \frac{100}{1} \\ = \scriptsize 12 \%$$

% by mass of oxygen = $$\frac{48}{100} \: \times \; \frac{100}{1} \\ = \scriptsize 48 \%$$

Note:

The results above can be interpreted as follows: 100 g of calcium trioxocarbonate (IV) contains 40g of calcium, 12 g of carbon, and 48g of oxygen. The percentage by mass of any element also represents its reacting mass in forming that compound.

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