It is possible to make this proximate determination because, as shown by Proust's Law of Constant Proportions, pure substances always have the same elements combined in the same mass ratio. Furthermore, this is an intensive property, that is, it does not depend on the sample quantity.
Many researched substances, such as those in which it is necessary to know if there is an active principle that can fight a disease, are often unknown. Thus, chemists send samples of these substances to laboratories or official bodies, which carry out this analysis and later receive the results back, such as the percentage composition by mass of that substance. The most commonly performed analysis on unknown organic compounds is the combustion analysis.
The percentage formula is important as it is the starting point for determining the other chemical formulas of compounds, such as the minimum or empirical formula and the molecular formula.
The mathematical formula used to calculate this percentage is given by:
The rule of three is also used to perform these calculations. See the example below:
"Determine the percentage formula of an inorganic salt, and analysis of your sample indicated that in 50 g of this substance there are 20 g of calcium, 6 g of carbon and 24 g of oxygen."
Resolution:
- Using the formula:
Calcium mass percentage = mass of calcium in the sample. 100%
total sample mass
Calcium mass percentage = 20 g. 100%
50 g
Calcium mass percentage = 40%
Percentage of carbon mass = 6 g. 100%
50 g
Percentage of carbon mass = 12%
Oxygen mass percentage = 24 g. 100%
50 g
Oxygen mass percentage = 48%
Thus, the proximate formula can be expressed by:Here40%Ç12%O48%
- This same calculation can be done by rule of three, taking a 100 g sample:
Ca mass substance
50 g 20 g of Ca
100g x
x = 40 g Ca in 100 g sample or 40% Ca.
C mass substance
50 g 6 g of C
100 g y
y = 12 g Ca in 100 g sample or 12% C.
mass substance of O
50 g 24 g of O
100 g w
w = 48 g Ca in 100 g sample or 48% O.
By Jennifer Fogaça
Graduated in Chemistry
Source: Brazil School - https://brasilescola.uol.com.br/quimica/formula-percentual-ou-centesimal.htm