Standard formula for calculating mass percentage in textbooks

Similar to calculating percentages for goods or interest rates, the method for calculating mass percentage in Chemistry applies to elements in compounds and mixtures. Below is a comprehensive document on calculating mass percentage, students should refer to for quick and easy review of concepts and mastering Chemistry exercises for grades 10 and 11.

Formula for calculating mass percentage of metals, compounds in mixtures,...

1. Method for calculating mass percentage when mass is known

Step 1. Calculate the mass percentage composition of elements in the mixture

Formula for calculating mass percentage:

Mass percentage = (mass of substance/total mass of mixture) x 100%.

Where:
- Mass of substance: Will be provided in the problem. If not mentioned, refer to the method for calculating mass percentage when mass is unknown below.
- Total mass of mixture is the combined mass of the substances forming that mixture.

Step 2. Calculate the total mass of the mixture

In cases where the mass of the compound or element is known, simply add them together to obtain the total mass of the mixture/solution. This is the general formula for calculating mass percentage.

Example 1: Calculate the mass percentage of 5g sodium hydroxide when dissolved in 100g water.
=> Sodium Hydroxide in the problem is 5g, water is 100g so the total mass of the mixture is 105g.

Example 2: Calculate the mass of Sodium Chloride and water needed to form a 175g solution with a concentration of 15%.
=> When you know the mass percentage is 15% and the total mass of the mixture is 175g.

Step 3. Determine the mass of the substance for which the mass percentage is required

The problem requires calculating the mass percentage of a specific substance, which means you need to find the mass of that substance in proportion to the total mass of all components. Write the mass of the substance to find the mass percentage. That is the numerator in the mass percentage formula.

Example 1: The mass of Sodium Hydroxide is 5g (substance requiring mass percentage).
Example 2: The mass of the substance requiring mass percentage is unknown, you need to find it.

Step 4. Substitute the variables into the equation for calculating mass percentage

Once you have determined the value of each variable, simply replace them correctly into the equation for calculating mass percentage in step 1.

Example 1: Mass percentage = 5/105 x 100
Example 2: Rearrange the equation for calculating mass percentage to find the mass of the unknown substance.
Mass of substance = (mass percentage x total mass of mixture) : 100 = 15 x 175 : 100

Note: Apart from revising mass percentage calculation methods, the formula for mass percentage, chemistry encompasses many other valuable concepts to remember. And calculating the molarity of solutions and compounds is one of them. In the following article, Mytour will share with you how to calculate Mol concentration and detailed example exercises, so be sure to refer to it to excel in your chemistry studies!

Step 5. Calculate the mass percentage

Once the equation is fully adjusted, you simply calculate the mass percentage. You just need to multiply the mass of the substance by 100 and divide by the mass of the mixture to get the mass percentage of that substance.

Example 1: 5/105 x 100 = 4.761 (%). Thus, the mass percentage of 5g Sodium Hydroxide in 100g water is 4.761%.
Example 2: When rearranging the equation to find the mass of the substance, it becomes 15 x 175 : 100 = 26.25 (g) Sodium Chloride. At this point, the mass of water added is the mixture minus this, 175 - 26.25 = 148.75 (grams).

2. Method for calculating mass percentage when mass of elements is unknown

Step 1. Determine the equation for calculating mass percentage in compounds

Formula:

Calculate mass percentage = (atomic mass of element/molecular mass of compound) x 100%.

Chemical compound formula: AxByCz.

Step 1. Define the mass percentage equation for compounds

In which:

- Atomic mass of the element is the mass of 1 mole of the element.

- Molecular mass is the mass of 1 mole of the compound.

Utilize the periodic table to lookup the atomic weight of elements. The atomic mass will be listed below the chemical symbol. Then, you need to write the atomic weight of the element in that compound.
Example 1: Oxygen has an atomic mass of 16, Hydrogen has an atomic mass of 1.
Example 2: Carbon has an atomic mass of 12, Oxygen has an atomic mass of 16, and Hydrogen has an atomic mass of 1.

Step 4. Multiply the atomic weight by the number of moles

Once you determine the number of moles of elements in the compound, that ratio will be represented by the small numbers beneath the chemical formula of the compound.

Example 1: Water: H₂O
- H₂ = 1 x 2 = 2.
- O = 16.

Example 2: Glucose: C₆H₁₂O₆
- C₆ = 12 x 6 = 72.
- H₁₂ = 1 x 12 = 12.
- O₆ = 16 x 6 = 96.

Step 5. Calculate the total mass of the compound

You sum the masses of the elements in the compound, then calculate the total mass of the compound based on the mass calculated per mole ratio.

Example 1: Mass of compound H₂O = 2 + 16 = 18 (g/mol).
Example 2: Mass of compound Glucose C₆H₁₂O₆ = 72 + 12 + 96 = 180 (g/mol).

Step 6. Determine the mass of the element needed to calculate the percentage by mass

The task requires calculating the mass percentage, meaning you will need to find the specific mass of the element in the compound with the percentage ratio of the total mass of all those elements. Once identified, note down those elemental masses.

Example 1: The mass of H in the compound water H₂O is 2 g/mol.
Example 2: The mass of C in the compound Glucose is 72g/mol.

Step 7. Substitute the variables into the mass percentage equation

Once you have determined the variable values, you can substitute them into the formula to calculate the mass percentage.

Example 1: % mass = 2/18 x 100.
Example 2: % mass = 72/180 x 100.

Step 8. Calculate the mass percentage

After substituting the numbers into the formula, simply calculate to obtain the mass percentage of the element in the compound.

Example 1: % mass = 2/18 x 100 = 11.1%.
Example 2: % mass = 72/180 x 100 = 40%.

Exercise: Calculate the mass percentage of the element present in the compound Na₂O.

Solution:

In 1 mol of Na₂O, there are 2 moles of Na atoms and 1 mole of O atoms. Therefore, the mass percentage of the elements in Na₂O is:
- %m_Na = (2 x 23/(23x2 + 16)) x 100 = 74.2%.
- %m_O = (16/(23x2 + 16)) x 100 = 25.8%.

With the method of calculating mass percentage shared here by Mytour, it is hoped that readers can easily enhance their understanding of percentage mass calculation in Chemistry.