Tabla de Contenidos
In order to calculate the percentage by mass of an element in a compound, we must perform the following steps:
- Divide the mass of the element in 1 mole of the compound by the molar mass of the compound.
- Multiply the result obtained by 100.
The formula to calculate the percentage by mass is:
(mass of component / total mass) x 100 = percentage by mass
Another equivalent equation that can be used in solutions is:
(mass of solute / mass of solution) x 100 = percent by mass
Mass percent problem example
To better understand what the mass percent method is and learn how to calculate it, let’s look at the following problem as an example. In this case, we will try to find out what are the mass percentages of hydrogen and oxygen in an element as everyday as water (H 2 O).
To correctly calculate the percent by mass and get the correct answer, you need to perform these steps:
- Find the mass of each atom
With the help of a periodic table, we will find the atomic masses of hydrogen and oxygen. Once we find these elements, we will see that:
The mass of hydrogen (H) is 1.008 g/mol
The mass of oxygen (O) is 16.00 g/mol
- Calculate the number of grams of each component in one mole of the solution.
Therefore, here we need to find out how many grams of oxygen and hydrogen are in one mole of water (H 2 O). Since water contains 2 moles of hydrogen atoms and 1 mole of oxygen atoms, we must perform the following calculations:
2.02 g (2 moles per 1.008 grams per mole ) of hydrogen (H)
16.00 g (16.00 grams per mole) of oxygen (O)
In this way, we will know that the mass of a mole of water (H 2 O) is:
2.02g + 16.00g = 18.02g
- Find the percentage by mass of each atom
Finally, we will apply the formula to obtain the percentage by mass:
(mass of component / mass of total) x 100 = percentage by mass
Thus we will obtain the percentages by mass of each element:
For Hydrogen:
% by mass (H) = (mass of 2 moles of hydrogen / mass of 1 mole of H2O) x 100
% by mass (H) = (2.02 g / 18.02 g) x 100
% by mass (H) = 11.21%
For Oxygen:
% by mass of (O) = (mass of 1 mol of oxygen / mass of 1 mol of H2O ) x 100
% by mass of (O) = (16.00 g / 18.02 g) x 100
% by mass of (OR) = 88.79%
- Mass percent result
After following the steps above, we will then know the mass percentage of hydrogen and oxygen in the water.
% by mass of hydrogen (H) = 11.21%
% by mass of oxygen (O) = 88.79%
- control the result
To know that the calculation of the percentage by mass has been carried out correctly, it is necessary to verify that the percentages that were obtained give a total of 100%.
11.21% + 88.79% = 100.00%
In this case, as the sum of the data obtained is 100%, we verify that the calculation has been carried out correctly.
Bibliography
Patiño, A. Introduction to chemical engineering: mass and energy balances. Volume II. (2000). Mexico. UIA.
Science Blog III. Exercises of mass percentage and volume percentage. Chemistry. Available at: http://quimiciencia3.blogspot.com/p/blog-page_14.html
Professor 10 of Maths. Solutions exercises and solved problems . Chemistry. Available at: https://www.profesor10demates.com/2013/04/disoluciones-ejercicios-y-problemas_28.html
