Atomic Mass and Molecular Mass – Definition, Different Elements and Solved Example

Atomic Mass and Molecular Mass:

Definition of Atomic Mass?

Atomic mass is the mass of an atom. It is measured in atomic mass units (amu). One atomic mass unit is equal to 1/12 the mass of a carbon-12 atom.

Atomic mass refers to the average mass of an atom of a specific chemical element. It is a fundamental property of atoms and is often expressed in atomic mass units (amu) or unified atomic mass units (u). Atomic mass takes into account the masses of the protons, neutrons, and electrons that constitute an atom.

The atomic mass of an element is primarily determined by the combined mass of its protons and neutrons, as the mass of an electron is considered negligible in comparison. Since different elements can have varying numbers of neutrons, which are responsible for the isotopes of an element, the atomic mass is an average value that considers the abundance of each isotope in a naturally occurring sample.

Definition of Molecular Mass

Molecular mass refers to the sum of the atomic masses of all the atoms present in a molecule. It represents the mass of a specific arrangement of atoms within a compound and is often expressed in atomic mass units (amu) or grams per mole (g/mol).

To calculate the molecular mass of a compound, one needs to determine the atomic masses of the individual atoms in the molecule. This information can be obtained from the periodic table, where the atomic masses of different elements are listed. By adding up the atomic masses of all the atoms in the molecule, accounting for the number of each type of atom as indicated by the chemical formula, the molecular mass can be determined.

Molecular mass is crucial in various aspects of chemistry. It is used to calculate the molar mass of substances, which allows for conversions between mass, moles, and particles. The molar mass serves as a conversion factor in stoichiometry, enabling the determination of the quantities of substances involved in chemical reactions.

 

Different Elements and their Atomic Mass and Molecular Mass:

Here are some examples of different elements and their respective atomic masses and molecular masses:

1. Hydrogen (H):
   • Atomic Mass: Approximately 1.008 amu
   • Molecular Mass: 2.016 amu (diatomic molecule, H2)
2. Oxygen (O):
   • Atomic Mass: Approximately 15.999 amu
   • Molecular Mass: 31.998 amu (diatomic molecule, O2)
3. Carbon (C):
   • Atomic Mass: Approximately 12.011 amu
   • Molecular Mass: 12.011 amu (elemental form, C)
4. Nitrogen (N):
   • Atomic Mass: Approximately 14.007 amu
   • Molecular Mass: 28.014 amu (diatomic molecule, N2)
5. Sodium (Na):
   • Atomic Mass: Approximately 22.990 amu
   • Molecular Mass: Not applicable (as an element, exists as single atoms)
6. Chlorine (Cl):
   • Atomic Mass: Approximately 35.453 amu
   • Molecular Mass: 70.906 amu (diatomic molecule, Cl2)
7. Calcium (Ca):
   • Atomic Mass: Approximately 40.078 amu
   • Molecular Mass: Not applicable (as an element, exists as single atoms)
8. Water (H2O):
   • Molecular Mass: 18.015 amu (H2O consists of two hydrogen atoms with atomic mass 1.008 amu each, and one oxygen atom with atomic mass 15.999 amu)
9. Carbon Dioxide (CO2):
   • Molecular Mass: 44.01 amu (CO2 consists of one carbon atom with atomic mass 12.011 amu and two oxygen atoms with atomic mass 15.999 amu each)

These values represent approximate atomic masses and molecular masses, as the atomic masses may vary slightly depending on the isotopic composition of the element. The molecular masses provided are for the most common forms of the compounds mentioned.

 

Solved Example of Atomic Mass and Molecular Mass:

Here’s a solved example that demonstrates the calculation of atomic mass and molecular mass:

Example: Calculate the atomic mass and molecular mass of water (H2O). Given the atomic masses: H = 1.008 g/mol and O = 16.00 g/mol.

Solution: To calculate the atomic mass of water (H2O), we need to consider the atomic masses of hydrogen (H) and oxygen (O) atoms in the molecule.

Atomic mass of water (H2O): H2O = 2(H) + 1(O) = 2(1.008 g/mol) + 1(16.00 g/mol) = 2.016 g/mol + 16.00 g/mol = 18.016 g/mol

Therefore, the atomic mass of water (H2O) is 18.016 g/mol.

To calculate the molecular mass of water, we sum up the atomic masses of all the atoms present in the molecule.

Molecular mass of water (H2O): H2O = 2(H) + 1(O) = 2(1.008 g/mol) + 1(16.00 g/mol) = 2.016 g/mol + 16.00 g/mol = 18.016 g/mol

Therefore, the molecular mass of water (H2O) is also 18.016 g/mol.

In this example, we calculated both the atomic mass and molecular mass of water (H2O) using the given atomic masses of hydrogen (H) and oxygen (O). The atomic mass represents the mass of one atom of an element, while the molecular mass represents the mass of one molecule of a compound.