3.1.2 Relative Formula Mass

In this lesson, we will explore how to calculate relative formula mass and use it to determine the percentage by mass in a compound.

Relative Formula Mass (Mr)

Relative formula mass (Mr) is used to determine the mass of compounds. It is the sum of the relative atomic masses of the atoms present in the formula of a compound. 

The relative formula mass of a compound is calculated by adding up the relative atomic masses of the atoms present in the compound according to their respective quantities shown in the formula. The relative atomic masses can be found in the periodic table.

Calculating Relative Formula Mass

To calculate the relative formula mass of a compound, follow these steps:

1. Identify the elements present in the compound.
2. Determine the number of atoms of each element in the formula.
3. Look up the relative atomic masses of each element in the periodic table.
4. Multiply the relative atomic mass of each element by the number of atoms of that element in the formula.
5. Sum up the results to obtain the relative formula mass of the compound.

Example: Calculating the relative formula mass of carbon dioxide (CO₂)

• Carbon (C): Relative atomic mass = 12.01, Number of atoms = 1
• Oxygen (O): Relative atomic mass = 16.00, Number of atoms = 2

Relative formula mass of CO₂ = (12.01 x 1) + (16.00 x 2) = 12.01 + 32.00 = 44.01

Balanced Chemical Equations and Relative Formula Mass

In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown is equal to the sum of the relative formula masses of the products in the quantities shown. This principle follows the law of conservationThe professional care, preservation, and restoration of archaeological materials and sites, often requiring scientific expertise. of mass.

Calculating Percentage by Mass

Percentage by mass represents the proportion of the mass of a particular element in a compound. It can be calculated using the following formula:

Percentage by mass = (Relative atomic mass of the element / Relative formula mass of the compound) x 100%

Example: Calculating the percentage by mass of oxygen in carbon dioxide (CO₂)

• Oxygen (O): Relative atomic mass = 16.00, Relative formula mass of CO₂ = 44.01

Percentage by mass of oxygen in CO₂ = (16.00 / 44.01) x 100% = 36.36%

Conclusion

Relative formula mass allows us to determine the mass of compounds by summing the relative atomic masses of the atoms present in the formula. It is important to balance chemical equations to ensure that the sum of the relative formula masses of the reactants is equal to the sum of the relative formula masses of the products. Percentage by mass helps us determine the proportion of the mass of a specific element in a compound.