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Formula Unit Mass

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Last updated date: 29th Feb 2024
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Molecular Unit Mass and Formula Unit Mass: An Introduction

The mass of an atom/molecule can be determined by mass spectroscopy. The macroscopic objects can be calculated in units of kilograms, tons, etc., whereas the microscopic particles can be calculated in atomic mass units. Hydrogen is taken as the reference mass to determine the mass of multi-electron atoms because of having a unity mass. Later, carbon having atomic mass 12 is used as a reference due to the abundance of the atom in the environment. The carbon having atomic mass 12 is used because carbon having atomic mass 13 and 14 are radioactive atoms.

The molecular mass unit can be defined as the molecule's mass, formulated from the molecular formula. The formula unit mass is the mass of the chemical formula of the molecule, which shows the number of atoms present in the molecule. The atomic mass is used to formulate the formula mass of the chemical formula.

What is a Formula Mass Unit?

The formula mass unit is the mass of the chemical formula of the compound present. The formula mass unit can be formulated from the addition of the number of moles of the atom present in the compound. The formula mass unit has a unit of amu similar to the molecular mass unit. The calculated mass of the formula mass unit can be the same or different from the molecular mass unit. The formula mass unit does not depend upon the attraction between the molecules. The formula mass unit depends upon the empirical formula of the compound.

Formula Mass Definition

The mass of the atom can be used to calculate the formula mass unit. The formula mass depends upon the number of moles of atoms involved and not on the attractive force involved in forming a dimer or trimer, but the molecular mass unit depends on the attractive force such as forming a dimer or trimer, etc.

A formula mass unit can be defined as the addition of the atomic masses of the atoms involved in the compound. Generally, the formula mass unit is used to formulate the mass of the compounds made up of ions, i.e., ionic compounds.

The unit of the formula mass unit is the atomic mass unit (amu) or um (unified mass). The atomic mass unit can be defined as the mass of the twelfth part of the carbon having atomic mass 12. Earlier, hydrogen was used as the reference to find out the mass of the other multi-electron atoms as they have an atomic mass of unity. But sometimes, carbon has atomic mass 12 to reference atomic mass because of the abundance of the carbon-atom present in the surroundings.


Formula Mass Unit Example

Formula mass units can be formulated by adding the mass of the number of moles of the atom present in the compound. The formula mass unit is based upon the empirical formula of the compound.

Now, we will find the formula mass unit of the ionic compound such as NaCl. The ionic compounds are those which are made up of the ions such as cations and anions in which one ion is positively charged, and the other ion is an anion, respectively.

The ionic compound having cation Na+ and the anion Cl- is involved.

The formula mass unit of the NaCl compound is-

Formula Mass Unit = Number of Moles x Atomic Mass of Na + Number of Moles x Atomic Mass of Cl


Formula mass unit = 1X23 + 1 X 35.5 = 58.5u

The formula mass unit of the NaCl is 58.5u or 58.5 amu.

Interesting Facts

  • Mass is used to measure the weight of an object.

  • A formula unit is the chemical formula of an ionic compound that lists the ions in the lowest ratio that equals a neutral electric charge.

  • The chemical formula is used to describe the elements or molecules that make up a substance.

Key Features of Formula Unit Mass

  • The unit of the formula mass unit is the atomic mass unit.

  • The molecular mass unit is the mass of the molecular mass, whereas the formula mass unit is the mass of the chemical formula concluded.

  • The formula unit mass can be formulated by adding the atomic mass of the number of moles of the atom present in the formula.

  • Macroscopic particles are those particles which are visible to the naked eye, whereas microscopic particles are those which are not visible to the naked eye.

FAQs on Formula Unit Mass

1. What do you mean by the atomic mass unit?

Atomic mass unit can be defined as the mass of the 12 parts of the carbon having an atomic mass of 12. The atomic mass unit can be designated as amu or um (unit mass).

2. What do you mean by chemical formula? 

Chemical formula can be defined as the composition of the atom present in the compound present. 

3. What is the unit of the formula mass unit?

The unit of the formula mass unit is the atomic mass unit. The atomic mass unit used carbon-12 as the reference for finding the atomic mass of the multi-electron atoms.