What is Molecular Mass: Definition, Example
In chemistry, molecular mass is an important property of molecules that are concerned with their physical and chemical properties. It is a measure of the total mass of all the atoms in a molecule.
Understanding molecular mass is an important concept in chemistry that helps us understand the behavior of different molecules in different chemical reactions.
In this tutorial, we will understand what molecular mass is, how to calculate it, and its applications in chemistry.
Definition of Molecular Mass
Molecular mass, also known as molecular weight, is the sum of atomic masses of the elements present in a molecule. It is a number that indicates how many times one molecule of a substance is heavier than 1/12th of the mass of one atom of carbon-12. Thus,
Molecular mass (M) = Mass of one molecule of the substance / 1/12th mass of one atom of carbon-12
Molecular mass is expressed in atomic mass units (u) or grams per mole (g/mol). It is calculated by adding up the atomic masses of all the atoms in a molecule.
For example, one molecule of water consists of two atoms of hydrogen and one atom of oxygen. The molecular mass of water (H2O) is 18.015 u. This is calculated by adding up the atomic masses of two hydrogen atoms (1.008 u each) and one oxygen atom (15.999 u). Thus,
Molecular mass of H2O = 2 × atomic mass of hydrogen + 1 × atomic mass of oxygen
= 2 (1.008 u) + 16.00 u = 18.02 u
It is obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together. For example, molecular mass of methane (CH4), which contains one carbon atom and four hydrogen atoms, can be obtained as follows:
Molecular mass of methane, (CH4) = (12.011 u) + 4 (1.008 u) = 16.043 u
Molecular Mass Calculation Examples
Example 1: Calculate the molecular mass of sulphuric acid whose molecular formula is H2SO4.
Solution:
Sulphuric acid (H2SO4) is a simple molecule that comprises two hydrogen atoms, one sulphur atom, and four oxygen atoms. So, we can calculate the molecular mass of sulphuric acid as follows:
Molecular mass of sulphuric acid (H2SO4) = (2 × atomic mass of hydrogen) + (1 × atomic mass of sulphur ) + (4 × atomic mass of oxygen)
= (2 × 1.008 u) + (1 × 32.00 u) + (4 × 16 u) = 2.016 + 32 + 64 = 98.016u
Example 2: Calculate the molecular mass of ethanol (C2H5OH).
Solution:
Ethanol (C2H5OH) is a more complex molecule that contains two atoms of carbon, six hydrogen atoms, and one oxygen atom. So, we can calculate the molecular mass of ethanol as:
Molecular mass of ethanol = (2 × atomic mass of carbon) + (6 × atomic mass of hydrogen) + (1 × atomic mass of oxygen)
= (2 × 12.011 u) + (6 × 1.008 u) + (1 × 15.999 u) = 46.069u
Gram-molecular Mass or Gram Molecule
When the molecular mass of a substance is expressed in grams, it is called gram-molecular mass or gram molecule. For example, the molecular mass of oxygen (O2) is 32 u. So, the gram molecular mass of oxygen is 32g.
Similarly, the molecular mass of chlorine (Cl2) = 2 * 35.5 = 71 u. So, gram molecular mass of chlorine = 71g.
Actual mass or absolute mass is the mass of one molecule of a substance. For example, the actual mass of one molecule of chlorine is equal to 71 x 1.66 x 10-24 g = 11.786 x 10-23 g.
How to calculate the number of gram molecules?
We can calculate the number of gram molecules of a substance present in a given mass of a substance by the following formula:
No. of gram molecules = Mass of a substance in grams / Molecular mass of the substance in grams
Example 1: Calculate the mass of 3.5 gram molecule of oxygen.
Solution: We know that,
No. of gram molecules = Mass of element in grams / Molecular mass of element in grams
So, no. of gram molecules = 3.5
Molecular mass of oxygen = 2 * 16 = 32g
Thus, mass of 3.5 gram molecule of oxygen = 3.5 * 32 = 112g
Example 2: Calculate the mass of 1.5 gram molecule of nitric acid (HNO3).
Solution:
No. of gram molecules = 1.5 (given)
Molecular mass of HNO3 = 1+ 14 + 3 x 16= 63u
Gram-molecular mass of HNO3 = 63g
Mass of 1.5 gram molecule of HNO3 = 63 x 1.5 = 94.5g
Example 3: Calculate the actual mass of one molecule of sulphuric acid (H2SO4 ).
Solution:
Molecular mass of H2SO4 = 2 x 1.008 + 32.00 + 4 x 16 = 98.016u
Since, 1u = 1.66 x 10-24 g
So, the actual mass of H2SO4 = 98.016 x 1.66 x 10-24 = 162.70 x 10-24g
Applications of Molecular Mass
(1) Determination of stoichiometry in chemical reactions:
One of the main applications of molecular mass is in the determination of stoichiometry in chemical reactions. Stoichiometry in chemistry is the calculation of the amount of reactants and products in a chemical reaction.
By knowing the molecular mass of a substance, we can calculate the amount of reactants and products involved in a chemical reaction. It also allows us to calculate the amount of product that will be produced from a given amount of reactants.
(2) Identification and characterization of unknown substances:
We can also use molecular mass in the identification and characterization of unknown substances. By determining the molecular mass of an unknown substance, we can compare it to known molecular masses to identify the substance. This is useful in forensic science, drug testing, and environmental analysis.
Google FAQs
1. What is the fundamental difference between molar mass and molecular mass?
Molar mass is the mass of one mole of a substance and we express it in grams per mole. While molecular mass is the mass of a single molecule or compound and we express it in atomic mass units.
2. How is molecular mass calculated?
We can determine the molecular mass by adding up the atomic masses of all the atoms in a molecule.
3. Why is molecular mass important?
Molecular mass is important because it helps to determine the stoichiometry of chemical reactions as well as to identify and characterize unknown substances.
4. How is molecular mass used in forensic science?
We use molecular mass in forensic science to identify unknown substances found at crime scenes. By determining the molecular mass of a substance, investigators can compare it to known molecular masses to identify the substance.
