Molecular Weights-Gram Molecular Masses

Molecular mass is defined as the mass of a given molecule measured in daltons. The related quantity relative molecular mass (defined by IUPAC) is the ratio of the mass of a molecule to the unified atomic mass unit. It is dimensionless. The molecular mass and relative molecular mass are different from each other but are related to the molar mass. 

What is Gram Molecular Mass?

The mass in grams of one mole of a molecular substance is defined as the gram molecular mass. It is the same as molar mass. However, gram molecular mass specifies the mass unit to be used.

• Different molecules of the same compound can have different molecular masses as there are many isotopes of an element.
• Molar mass is defined as the mass of a given substance divided by the amount of the substance (in g) and is expressed in g/mol. Molar mass is generally useful while dealing with macroscopic level quantities of a substance.
• Molecular mass is defined as the total sum of all the atomic masses of the atoms present in a molecule. The molecular mass gives the mass of a molecule relative to that of a 12C (carbon 12 atom), which is known to have a mass of 12.
• Molecular mass is a dimensionless quantity, but it is given the unit Da or amu (atomic mass unit) to depict the fact that its mass is relative to 1/12th of the mass of a single carbon-12 atom.
• Gram molecular mass is generally represented in grams or grams per mole (g/mol).

Gram Atomic Mass of Elements

The amount of an element measured in grams, which is numerically equal to the atomic mass of that element, is known as the gram atomic mass of that element.

The amount of a substance measured in grams, which is numerically equal to the molecular mass of that substance, is known as the gram molecular mass of that substance.

Gram molecular mass is different from the mass of one molecule of the same substance measured in grams and should not be confused with one another. The actual mass or molecular mass of a substance is defined as the mass of one molecule of that substance

Molar mass is the mass of 1 mol of atoms of an element. When it comes to a covalent molecular compound, the molar mass is the mass of 1 mol of molecules of that compound, while for an ionic compound, the molar mass is the mass of 1 mole of formula units. Thus, the molar mass of a substance is the mass (grams per mole) of 6.022 × 1023 atoms (Avogadro number), molecules, or units of that substance. For each case, the number of grams of a particular substance in 1 mol is equal to the number of atomic mass units that describe the formula mass, atomic mass, or molecular mass, respectively.

It can be calculated by taking an element’s atomic weight from the periodic table and then converting that numerical part into grams. Thus, when the mass of an element is written down in grams, it means it is the gram atomic mass of that element. This goes on to prove that the quantity of an element of a given substance, when weighed, is equal to the gram atomic mass which is also called one gram atom. 

Steps to Find Gram Molecular Mass

Use the following molecular formula to calculate mass:

1. Write down the relative atomic mass of each element of the molecule given in the formula.
2. Multiply the element’s atomic mass with the subscript after that element symbol (the number of atoms) in the molecular formula. If you find that there is no subscript, then it means that there is only one atom of that element in the molecule.
3. Add all of them and then the sum you get is the gram molecular mass.

Example

The gram atomic mass of Helium is 4 g. Sodium (Na) has an atomic weight equal to 22.99 u and a gram atomic mass equal to 22.99 grams. Thus, one mole of Sodium atoms is equal to 22.99 g. The mass of the H2O molecule is 18 atomic mass units, while the mass of 1 mole of water is equal to 18 g. So 18 g is also known as water’s gram molecular mass.

The molecular mass of N2 is equal to 28, so the gram molecular mass of N2 is also 28 g.

Similarly the molecular masses of:

          CH4 ⇒ 1 × 12 + 4 × 1 = 16 u

          H2 = 1 × 2 → 2 u.

          O2 =16 × 2 → 32 u.

          Cl2 = 35.5 × 2 → 71 u

Conclusion:

Molar mass is the smallest unit of a compound with one-twelfth of the mass of one carbon – 12 atoms. Gram molecular mass is the mass of one mole of a molecular substance in grams. It is the sum of all the molecular weight of each atom present in the molecule multiplied by its subscript. The unit used to measure is grams per mole.