Why Is Molecular Weight Important in Chemistry?
For students who are learning Chemistry, the term molecule is going to be one of the most frequently used terms from the day they start to learn the subject. Molecular weight can be defined as the average mass of a molecule of a compound compared to ¹/₁₂ the mass of carbon 12 (C - 12) and is calculated as the sum of the atomic weights of the constituent atoms, It is also called as molar mass. Eg: The molecular weight of water containing two atoms of hydrogen and one atom of oxygen, is 18 (i.e. 2 + 16). Also called molecular mass. An important aspect of molecules is the way their weight or mass is calculated in the form of molecular weight also referred to as molar mass.
The main question arises, What do we do by knowing the mass of such tiny atoms and molecules? Well, the significance of these atomic mass and molecular mass of a substance is huge in chemistry. In this topic, we have discussed molecular weight meaning, Let’s understand what is molecular mass.
Molecular Mass
The molecular mass of a compound is defined as the mass of one molecule. The molecular mass of a compound differs due to the presence of isotopes. Molecular mass is measured in AMU which stands for atomic mass units. In the above topics, we have discussed the molecular weight definition and molecular mass lets understand some key differences between them.
Difference between Molar Mass and Molecular Mass
Molar Mass Formula and Molecular Mass Formula
In this topic, we will discuss the difference between Molar Mass Formula and Molecular Mass Formula
Molecular Mass
Molecular mass is equal to the sum of the individual atomic mass of each atom in the molecule. By adding the atomic weight of the constituent elements of a molecule we can get molecular weight formula as:
Molecular Mass (AB2) = Atomic Mass of A + 2 × Atomic Mass of B
Molar Mass
Molar mass is equal to the sum of the individual atomic mass of each substance (in grams) in the molecule. By adding all the atomic weight of the elements given in grams we will get the molar mass equation as:
Molar Mass (ab2) = Atomic Mass (in Grams) of a + 2 × Atomic Mass of B (in Grams)
The Molecular Mass of the Molecule Can be Calculated by the Following Steps:
First Identify the formula of the compound or the molecule given.
Use the formula to determine the number of atoms present in each element of the compound or molecule.
Now multiply the atomic weight of each element by the number of atoms of that particular element. Similar to this do it for all the elements in the molecule or compound.
Now sum all the values obtained in the above step.
Then add the unit as grams/mole and we will get the molecular mass of the substance given.
Solved Examples
Q1: Find the molecular mass of the water molecule.
Solution: We know that the formula of the water molecule is H2O.
The atomic mass of hydrogen = 1 AMU
The atomic mass of oxygen = 16 AMU
So, Molar mass of water = 2 x 1 + 16 = 18 AMU.
Q2: Calculate the relative molar mass of the hydrogen molecule.
Solution: The hydrogen molecule is represented by H2.
The relative molecular mass of hydrogen molecules can be calculated as the sum of the relative atomic masses of two atoms of hydrogen.
i.e 1U + 1U = 2U.
Conclusion
Molar mass, molecular mass, and relative molar mass differ from each other in terms of the unit they are expressed in. It can be easily noticed that the atomic mass and the molar mass of carbon-12 are numerically equal. Molar mass represents the weight of a one-mole molecule in g/mol, and molecular mass represents the weight of one molecule in AMU and relative molar mass represents the weight of a single atom in terms of ununified mass.
This same relationship is between atomic and molar masses of elements, between molecular masses and molar masses of molecular substances, and between formula masses and molar masses of ionic compounds.
FAQs on Molecular Weight Explained: Formula, Calculation & Examples
1. What is molecular weight and how is it calculated?
Molecular weight, also known as molecular mass, is the sum of the atomic weights of all the atoms present in a single molecule of a substance. It is expressed in atomic mass units (amu). To calculate it, you need to know the chemical formula of the molecule and the atomic weight of each element from the periodic table. The formula is: Molecular Weight = Σ (Number of atoms of an element × Atomic weight of that element).
2. How do you calculate the molecular weight of water (H₂O) as an example?
To calculate the molecular weight of water (H₂O), follow these steps:
Step 1: Identify the elements and the number of atoms of each. In H₂O, there are 2 atoms of Hydrogen (H) and 1 atom of Oxygen (O).
Step 2: Find the atomic weight of each element. The atomic weight of H is approximately 1.008 amu, and the atomic weight of O is approximately 16.00 amu.
Step 3: Calculate the total weight. Molecular Weight of H₂O = (2 × 1.008 amu) + (1 × 16.00 amu) = 2.016 + 16.00 = 18.016 amu.
3. What is the step-by-step method to calculate the molecular mass of glucose (C₆H₁₂O₆)?
Calculating the molecular mass of a complex molecule like glucose (C₆H₁₂O₆) follows the same principle:
Identify Atoms: The formula has 6 Carbon (C) atoms, 12 Hydrogen (H) atoms, and 6 Oxygen (O) atoms.
Atomic Weights: C ≈ 12.011 amu, H ≈ 1.008 amu, O ≈ 15.999 amu.
Sum the Weights:
Mass from Carbon = 6 × 12.011 amu = 72.066 amu
Mass from Hydrogen = 12 × 1.008 amu = 12.096 amu
Mass from Oxygen = 6 × 15.999 amu = 95.994 amu
Total Molecular Mass: 72.066 + 12.096 + 95.994 = 180.156 amu.
4. What is the difference between molecular weight and molar mass?
While numerically similar, molecular weight and molar mass are different concepts. Molecular weight is the mass of a single molecule, measured in atomic mass units (amu). In contrast, molar mass is the mass of one mole (approximately 6.022 x 10²³ molecules) of a substance, measured in grams per mole (g/mol). For example, the molecular weight of H₂O is ~18 amu, while its molar mass is ~18 g/mol.
5. When should you use 'formula weight' instead of 'molecular weight'?
The term 'molecular weight' is best used for substances that exist as discrete molecules, typically covalent compounds like H₂O or CO₂. For ionic compounds like Sodium Chloride (NaCl), which form a crystal lattice rather than individual molecules, the term 'formula weight' is more accurate. It represents the sum of the atomic weights of the atoms in the compound's empirical formula. For NaCl, the formula weight is the sum of the atomic weights of one sodium atom and one chlorine atom (~58.44 amu).
6. Why is molecular weight considered a relative mass and not an absolute mass?
Molecular weight is a relative mass because it is calculated based on a standard reference point. The entire scale of atomic mass units (amu) is defined relative to the mass of a single Carbon-12 (¹²C) atom, which is assigned a mass of exactly 12 amu. Therefore, the atomic weight of any element, and consequently the molecular weight of any compound, expresses how heavy its atoms are in comparison to one-twelfth the mass of a Carbon-12 atom, not their absolute mass in grams or kilograms.
7. How do you find the molecular weight of sulphuric acid (H₂SO₄)?
To find the molecular weight of sulphuric acid (H₂SO₄), you sum the atomic weights of its constituent atoms:
Hydrogen (H): There are 2 atoms. Atomic weight ≈ 1.008 amu. Total = 2 × 1.008 = 2.016 amu.
Sulfur (S): There is 1 atom. Atomic weight ≈ 32.06 amu. Total = 1 × 32.06 = 32.06 amu.
Oxygen (O): There are 4 atoms. Atomic weight ≈ 16.00 amu. Total = 4 × 16.00 = 64.00 amu.
Total Molecular Weight: 2.016 + 32.06 + 64.00 = 98.076 amu.
8. How does knowing the molecular weight of a substance have practical applications in chemistry?
Knowing the molecular weight is fundamental for many practical applications in chemistry. It is essential for:
Stoichiometry: It allows chemists to convert between the mass of a substance and the number of moles, which is crucial for balancing chemical equations and predicting the amount of reactants needed or products formed in a reaction.
Solution Preparation: It is used to calculate the mass of a substance required to prepare a solution of a specific molar concentration (molarity).
Gas Laws: It helps in determining the density of a gas at a certain temperature and pressure using the Ideal Gas Law.
Substance Identification: Techniques like mass spectrometry determine the molecular weight of unknown compounds, which is a key step in identifying their chemical structure.
