Formula Mass

Formula mass is the sum of the atomic weight of all atoms in a molecule or compound. It can be expressed in atomic mass unit. It is used to obtain the mass of either a single molecule or a given number of molecules. Understanding the formula mass for atomic mass is important to write balanced chemical equations, determine the molar mass, and do stoichiometry calculations. This article explains the formula mass, its calculation, and its applications with examples.

What is Formula Mass?

Formula mass is the sum of atomic weights of atoms present in a molecule or formula, but the measurement is given in atomic mass units (AMU). It calculates the mass of any molecule, either a single molecule, or a larger number of molecules. The formula for the for the mass of a covalent compound is simply the molecular mass. For ionic substances, the procedure will go the same way.

Still, it would be wrong to use the term “molecular mass” for them because the formula of an ionic substance doesn’t display the composition of a molecule by itself. The formula mass concept is also related to the mole concept since it is a suitable method of knowing the mass of different substances and calculating the number of atoms, molecules, or compounds.

Types of Formula Mass for Different Compounds

The different types of Formula mass are as follows:

Formula mass for covalent compounds

The formula mass of covalent compounds is also known as molecular mass, which is the sum of the atomic weights of all atoms that is present in a molecule.

For example, in the case of chloroform (CHCl₃), its mass results from adding the masses of one carbon atom (12.01 am) and one hydrogen atom (1.01 amu), as well as three chlorine atoms (35.45 am) which is as follows:

Formula Mass of CHCl₃ = 1(12.01) + 1(1.01) + 3(35.45)

= 12.01 + 1.01 + 106.35 = 119.37 amu

Furthermore, the formula mass of ibuprofen (C₁₃H₁₈O₂) is the overall mass of the atomic weights: carbon adds up to 13 atoms, hydrogen has 18 atoms, and oxygen has 2 atoms:

Formula Mass of C₁₃H₁₈O₂ = 13(12.01) + 18(1.01) + 2(15.99)

= 156.13 + 18.18 + 31.97 = 206.28 amu

Formula mass is a valuable concept of chemistry, and it offers the conversion between the mass of an element in the sample and the number of molecules or moles.

Formula mass for Ionic compounds

The formula mass of an ionic compound is the total weight of the atoms or the ions present in its formula. These values are respectively computed as the averages of the atomic masses of every atom present in the compound formula. On the other hand, as ionic compounds do not show discrete molecules’ true composition structure, using “molecular mass” for them is wrong.

Thus, the compound’s formula mass is used, which is a combination of the atomic weights of the different ions in this compound’s formula. One example of this approach is the equation by which the formula mass of sodium chloride (NaCl) is calculated as the total atomic mass of one sodium atom and one chloride atom such as:

Formula Mass of NaCl = 1(22.99) + 1(35.45)

= 22.99 + 35.45 = 58.44 amu.

Relative Formula Mass

Relative molecular mass (M_r) is equal to the sums of the relative atomic masses or units formula of all atoms in one molecule. It is the ratio of the mass of a molecule or formula unit to the mass of a carbon-12 atom.

Calculation of Relative Formula Mass

To find a compound’s relative formula mass, you have to add up the mass of the atoms in the substance’s formula unit, taking into account the subscripts for each element involved.

For example, the relative formula mass of magnesium carbonate (MgCO₃) is calculated as follows:

Mg: 24 g/mol

C: 12 g/mol

O: 16 g/mol

Relative formula mass (M_r) of MgCO₃ = 24 + 12 + (3 x 16) = 84 g/mol

Gram Formula Mass

Gram formula mass (GFM) refers to the molar mass of one mole of an element, compound ion, or molecule of a substance written in grams. It characterizes the weight of a substance as equal to the same weight of its formula mass, which is taken in atomic mass units. Each element atom has its own mass; depending on that, the formula mass of each molecule is different. The formula mass of the compound is calculated by adding the atomic masses of constituent atoms of the compound.

Formula for Gram Formula Mass

The formula for calculating the Gram Formula Mass is:

Gram Formula Mass (G) = Solute Mass / Formula Mass of Solute

Where:

G is the Gram Formula Mass

Example of Gram Formula Mass Calculation

Example: Calculate the Gram Formula Mass of the compound MgSO₄

Solution:

1 atom of magnesium = 1 × 24 = 24g

1 atom of sulfur = 1 × 32 = 32g

4 atoms of oxygen = 4 × 16 = 64g

Gram Formula Mass = 24 + 32 + 64 = 120g/mol

FAQs on Formula mass

What is meant by formula mass?

The formula mass represents the sum of all the atomic masses that are written in a chemical formula.

What is 1 mole?

One mole is the amount equivalent to Avogadro’s number, which is defined as 6.022 x 10²³ atoms, molecules, or ions.

What is the mass formula unit?

The mass formula unit is the total mass of one formula unit of a substance, calculated by adding the atomic masses.

Why is formula mass used?

Formula mass helps calculate quantities in chemical reactions and understand the composition of substances.

What is formula mass vs molar mass?

Formula mass is the mass of one formula unit of a compound while molar mass is mass of 1 mole i.e. 6.023 × 10²³ moles of compounds