What Is Molecular Mass Definition Formula and How to Calculate It with Examples
Molecular mass is the mass of a given molecule. It is measured in Daltons. Molecular mass is most commonly used when referring to the mass of a single or specific well-defined molecule and less commonly than molecular weight when referring to a weighted average of a sample. The molecular mass of small to medium size molecules measured by mass spectrometry and it can be used to determine the composition of elements in the molecule or compound.
What is Molecular Mass?
Molecular mass is also called molecular weight. Molecular mass is the mass or weight of a molecule. Different molecules of the same compound may have different molecular masses because an element has different types of isotopes which are present in the compound. The related quantity of relative molecular mass is the ratio of the mass of a molecule to the united atomic mass. The molecular mass and relative molecular mass are distinct but related to molar mass. The molar mass is defined as the mass of a given substance divided by the number of moles of a substance. The unit of molar mass is g/mol.
How to Find Molecular Mass?
To find the molecular mass, find the atomic masses of each element by using a periodic table. Multiply the number of atoms with an element's atomic mass and add the masses of all elements in the molecule.
Formula of Molecular Mass/Molecular Mass Equation
The formula of molecular mass or molar mass is:
\[\text{molarmass} = \dfrac{{\text{mass}}}{{\text{number of moles}}}\]
How to Calculate Molecular Mass?
The total mass of a compound is called molecular mass or molecular weight. It is equal to the sum of the individual atomic masses of each element.
Molecular Mass Examples
There are some examples of molecular mass.
H2O: The atomic mass of hydrogen in the periodic table is 1u and the atomic mass of the oxygen atom is 18u. So, we can calculate the molecular mass of water molecules as follows:
\[{H_2}O = 2 \times 1 + 16 = 18u\]
Hence, the molecular mass of water molecules is 18u.
NH3: The atomic mass of Hydrogen in the periodic table is 1u and the atomic mass of nitrogen is 14u. So, we can calculate the molecular mass of ammonia molecule as follows:
\[N{H_3} = 3 \times 1 + 14 = 17u\]
Hence, the molecular mass of ammonia gas is 17u.
CaCO3: The atomic mass of calcium is 40u, the atomic mass of carbon is 12u, and the atomic mass of oxygen is 16u in the periodic table. So, we can calculate the molecular mass of calcium carbonate as follows:
\[CaC{O_3} = 40 + 12 + 3 \times 16 = 100u\]
Hence, the molecular mass of calcium carbonate is 100u.
CaCl2: The atomic mass of calcium is 40u and the atomic of chlorine is 35.45u in the periodic table. So, we can calculate the molecular mass of calcium chloride as follows:
\[CaC{l_2} = 40 + 2 \times 35.45 = 110.9u\]
Hence, the molecular mass of calcium chloride is 110.9u.
Molecular Mass of Compounds
The molecular mass of a compound can be found by using the following steps:
Determine the molecular formula of the compound.
Use the periodic table to determine the atomic mass of each element present in the compound.
Multiply each element’s atomic mass with the number of atoms that element in the compound. This number is denoted by the subscript next to the element symbol in the molecular formula.
Add these values together for each different atom.
The total value will be the molecular mass of the compound.
Molecular Mass of Elements
The molecular mass of an element is the sum of the atomic mass of atoms present in that element. Find the atomic mass of each element by using the given mass in the periodic table.
Interesting Facts
Molecular mass is defined as the sum of atomic masses of elements present in a molecule whereas molar mass is the ratio of the mass of the compound and the number of molecules present in the compound.
As the molecular weight increases, mechanical properties generally increase.
Key Features of Molecular Mass
The molecular weight of a polymer is directly related to its properties (strength, processability, brittleness).
Hydrogen has the smallest molecular mass.
Molecular mass is useful in analysing the results of experiments.
Molecular mass can be calculated by knowing the molecular formula.
FAQs on Molecular Mass Explained with Formula and Calculation
1. What is molecular mass?
The molecular mass is the sum of the atomic masses of all the atoms present in a molecule, expressed in atomic mass units (u).
- It is calculated by adding the relative atomic masses of each atom in the chemical formula.
- It is also called molecular weight in common usage.
- For example, the molecular mass of H2O is (2 × 1) + 16 = 18 u.
2. How do you calculate molecular mass?
To calculate molecular mass, add the atomic masses of all atoms in the molecule according to its chemical formula.
- Step 1: Write the correct chemical formula.
- Step 2: Note the atomic mass of each element from the periodic table.
- Step 3: Multiply each atomic mass by its subscript.
- Step 4: Add all the values.
3. What is the formula for molecular mass?
The formula for molecular mass is the sum of the relative atomic masses of all atoms in a molecule: Molecular Mass = Σ (atomic mass × number of atoms).
- It applies only to covalent molecules.
- Atomic masses are taken from the periodic table.
- The unit is atomic mass unit (u).
4. What is the difference between molecular mass and molar mass?
The molecular mass is the mass of one molecule in atomic mass units (u), while the molar mass is the mass of one mole of substance in grams per mole (g/mol).
- Molecular mass is expressed in u.
- Molar mass is expressed in g/mol.
- Numerically, they are equal but differ in units.
5. How is molecular mass related to moles?
The molar mass (numerically equal to molecular mass) is used to convert between mass and moles using the formula: moles = mass / molar mass.
- If you know the mass in grams, divide by molar mass (g/mol).
- This relationship is based on Avogadro’s number (6.022 × 1023).
6. What is the molecular mass of water (H2O)?
The molecular mass of H2O is 18 u.
- Hydrogen atomic mass = 1 u, and there are 2 hydrogen atoms.
- Oxygen atomic mass = 16 u.
- Total = (2 × 1) + 16 = 18 u.
7. What is the difference between molecular mass and formula mass?
The molecular mass refers to the mass of a discrete molecule, while the formula mass refers to the mass of an ionic compound’s formula unit.
- Molecular mass applies to covalent compounds like CO2.
- Formula mass applies to ionic compounds like NaCl.
- Example: Formula mass of NaCl = 23 + 35.5 = 58.5 u.
8. Why is molecular mass important in chemistry?
The molecular mass is important because it allows chemists to calculate moles, perform stoichiometric calculations, and predict reaction quantities.
- Used to convert between grams and moles.
- Essential for balancing chemical equations quantitatively.
- Helps determine empirical and molecular formulas.
9. Can you give an example of molecular mass calculation for glucose?
The molecular mass of C6H12O6 (glucose) is 180 u.
- Carbon: 6 × 12 = 72 u
- Hydrogen: 12 × 1 = 12 u
- Oxygen: 6 × 16 = 96 u
- Total = 72 + 12 + 96 = 180 u
10. What are common mistakes when calculating molecular mass?
Common mistakes in calculating molecular mass include ignoring subscripts, using incorrect atomic masses, or confusing it with molar mass units.
- Forgetting to multiply atomic mass by the subscript.
- Using rounded atomic masses incorrectly.
- Mixing up units (u vs g/mol).
- Misreading chemical formulas such as Ca(OH)2 (remember to multiply atoms inside brackets).
