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How to Find Percent

Updated on October 15, 2011

The amount in percent % of a substance can be calculated by dividing the its quantitative value by the total amount and multiplying by hundred. Here we will explain in terms of mathematical calculation and percentage composition of elements in chemical compounds.

Mathematical Calculation of Percentage

The simplest mathematical formula to find the percentage of an equation is to multiply the fraction by 100%. But the total value must be calculated and the subject divided by that total before multiplying by 100.

Consider the following Question,

What is the percentage of water in a mixture containing 20g of Water, 25g of Palm oil, and 5g of Sand?


1. list the contents and their amounts in the mixture.

Water = 20g, Palm Oil = 25g, and Sand = 5g.

2. Add the entire mass of the mixture to obtain the total mass of the mixture

Total mass of mixture = mass of water + mass of palm oil + mass of sand

= 20 + 25 + 5 = 50g

3. Divide the mass of water by the total mass and multiply by 100%

= 20/50 × 100%

= 40%

Percentage composition in chemical compounds

The percentage of each element in a chemical compound can be calculated by substituting the relative molecular mass of each of the elements in the molecular formula of the compound. The implication of this is that you should either have to memorise the relative atomic mass of each of the elements or might simply find them attached to the question.

To calculate the percentage composition of o in the compound y2xo4, with the relative molecular mass of y as 1, x 32, and o 16, we must calculate the total value of y, x, and o in the chemical formula. i.e.

y = 2 × 1 = 2, x = 32, and o = 4 × 16 = 64

Hence, total amount is equal to 2 + 32 + 64 = 98.

Percentage composition of Carbon in Ethanol

The percentage in grams of Carbon can be calculated by first writing the molecular formula of the Ethanol.

Molecular formula of Ethanol - C2H5OH

Then, the relative molecular mass of each element is listed to find the relative atomic mass of Ethanol.

Relative atomic mass of Carbon - 12, Hydrogen - 1, Oxygen 16.

Next, the relative atomic mass of the elements are substituted in the molecular formula to obtain the total molecular mass of the Ethanol, as follows:

2(12) + 5(1) + 16 + 1 = 24 + 5 +16 + 1

= 46.

Finally, the value of the total relative atomic mass of the Carbon (2 × 12) is divided by the molar mass of the Ethanol and expressed in Percentage.

24/46 × 100%

= 52.17% of Carbon

The implication is that over 50% of the content of Ethanol is carbon.


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