Molecular Weight can be defined as the sum of the atomic masses of all the elements present in a molecule. It is represented in the atomic mass unit (amu or u). The molecular weight also tells us the number of grams of a substance present in one mole of that compound. Hence, it needs to be the total atomic weight of every atom that is present in a particular molecule of any specific chemical compound. For example, in the case of Oxygen (O2), the molecular weight will be 32, that is the sum of the atomic weights in a single Oxygen atom.
| Table of Contents |
Key Terms- Molecular weight, Molecule, atom, Molecular Formula, Chemical component, atomic weight.
What is Molecular Formula?
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Molecular formula is the representation of the atoms present and the number of those atoms in a given molecule. The atomic symbol and the subscript below it indicate the number of atoms of that element present in a molecule. For Example: Molecular formula of oxygen is O2 and Carbon is C4.
Molecular Formula of Propanol
Calculation of Molecular Weight
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Molecular weight of an element can be simply calculated by multiplying the number of each type of atom present in a molecule by the atomic weight and adding the weights of other atoms.
For instance, the molecular weight of NaCl can be calculated as,
Molecular Weight of Na = 23
Molecular Weight of Cl = 35
No. of Na molecules = 1
No. of Ca molecules = 1
Hence, total molecular weight of NaCl = 23(1) + 35(1) = 58
NaCl Molecular Weight Calculation
Solved Examples for Molecular Weight Calculation
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Here are some solved examples of molecular weight calculations-
Example 1. Find out the Molecular Weight of Methane(CH4).
Ans. In Methane, there are one Carbon atom and 4 Hydrogen atoms.
We know that, atomic mass of C is 12.011u
Atomic mass of H is 1.0084u
Hence, the Molecular Weight of Methane will be-
12.011u + (4 x 1.0084)u
= 12.011u + 4.032u
= 16.043 g/mol
Example 2. Find out the Molecular Weight of glucose (C6H12O6).
Ans. In Glucose, there are 6 Carbon atoms, 12 Hydrogen atoms, and 6 Oxygen atoms.
We know that, atomic mass of C is 12.011u
Atomic mass of H is 1.0084u
Atomic mass of O is 15.999u
Hence, the molecular weight of Glucose will be-
(6 x 12.011)u + (12 x 1.0084)u + (6 x 15.999)u
= 72.066u + 12.096u + 95.994u
= 180.156 g/mol
Difference Between Molecular Weight and Molecular Mass
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The difference between molecular weight and molecular mass is the same as the difference between the conventional weight and the mass of an object.
Molecular weight is the force acting on a molecule with a certain molecular mass. As molecules are very tiny, this force is also minute.Therefore, molecular mass and molecular weight are often used interchangeably.
Here are the differences between molecular weight and molecular mass-
| Molecular Weight | Molecular Mass |
|---|---|
| Atomic mass is the sum of the masses of all the atomic units of an atom. | The molecular mass is the sum of the atomic masses of all the elements present in a molecule. |
Things To Remember
- The sum of the atomic masses of all the atoms present in a molecule is called the molecular mass or molecular weight.
- 1 atomic mass unit is one-twelfth the weight of the carbon-12 isotope. It is represented as amu or u.
- Formula unit mass is calculated for substances that have constituent elements as ions. It is calculated just like the molecular mass.
- For any molecule, the representation of the atoms present in it along with the number of these atoms is called the molecular formula.
Sample Questions
Ques1. Calculate the molecular masses of H2O and HNO3. (3 Marks)
Ans. Atomic mass of hydrogen (H) = 1 u
Atomic mass of O = 16 u
Atomic mass of N = 14 u
- Molecular mass of H2O = (2 × atomic mass of H) + atomic mass of O = (2 × 1) + 16 = 2 + 16 = 18 u
- Molecular mass of HNO3 = atomic mass of H + atomic mass of N + (3 × atomic mass of O) = 1 + 14 + (3 × 16) = 15 + 48 = 63 u
The molecular masses of H2O and HNO3 are 18 u and 63 u respectively.
Ques2. Calculate the formula unit masses of CaCl2. (3 Marks)
Ans. Atomic mass of Cl = 35.5
Atomic mass of Ca = 40
Formula unit of CaCl2 = atomic mass of Ca + (2 × atomic mass of Cl) = 40 + (2 × 35.5) = 40 + 71 = 111 u
So, the formula unit mass of CaCl2 is 111 u respectively.
Ques3. Calculate the molecular masses of H2, Cl2, O2, CO2, NH3, CH4, C2H4 and CH3OH. (5 Marks)
Ans. Atomic mass of H = 1 u
Atomic mass of Cl = 35.5 u
Atomic mass of O = 16 u
Atomic mass of N = 14 u
Atomic mass of C = 12 u
- Molecular mass of H2 = 2 × atomic mass of H = 2 u
- Molecular mass of Cl2 = 2 × atomic mass of Cl = 71 u
- Molecular mass of O2 = 2 × atomic mass of O = 32 u
- Molecular mass of CO2 = atomic mass of C + (2 × atomic mass of O) = 12 + (2 × 16) = 12 + 32 = 44 u
- Molecular mass of NH3 = atomic mass of N + (3 × atomic mass of H) = 14 + (3 × 1) = 14 + 3 = 17 u
- Molecular mass of CH4 = atomic mass of C + (4 × atomic mass of H) = 12 + (4 × 1) = 12 + 4 = 16 u
- Molecular mass of C2H4 = (2 × atomic mass of C) + (4 × atomic mass of H) = (2 × 12) + (4 × 1) = 24 + 4 = 28 u
- Molecular mass of CH4OH = atomic mass of C + (4 × atomic mass of H) + atomic mass of O = 12 + (4 × 1) + 16 = 28 + 4 = 32 u
Ques4. Calculate the formula unit masses of Na2O, ZnO and K2CO3. (4 Marks)
Ans. Atomic mass of Na = 23
Atomic mass of O = 16
Atomic mass of Zn = 65
Atomic mass of K = 39
Atomic mass of C = 12
- Formula unit mass of Na2O = (2 × atomic mass of Na) + atomic mass of O = (2 × 23) + 16 = 62 u
- Formula unit mass of ZnO = atomic mass of Zn + atomic mass of O = 65 + 16 = 81 u
- Formula unit mass of K2CO3 = (2 × atomic mass of K) + atomic mass of C + (3 × atomic mass of O) = (2 × 39) + 12 + (3 × 16) = 138 u
Ques5. Calculate the molar mass of these substances: ethyne, sulphur molecule, phosphorus molecule, and hydrochloric acid. (3 Marks)
Ans. Atomic mass of C = 12
Atomic mass of H = 1
Atomic mass of S = 32
Atomic mass of P = 31
Atomic mass of Cl = 35.5
- Molar mass of ethylene, C2H2 = (2 × atomic mass of C) + (2 × atomic mass of H) = (2 × 12) + (2 × 1) = 24 + 2 = 26 g/mol
- Molar mass of sulphur molecule, S8 = 8 × atomic mass of S = 8 × 32 = 256 g/mol
- Molar mass of phosphorus molecule, P4 = 4 × atomic mass of P = 4 × 31 = 124 g/mol
- Molar mass of hydrochloric acid, HCl = atomic mass of H + atomic mass of Cl = 1 + 35.5 = 36.5 g/mol
Ques6. How to calculate the molecular weight of a molecule? (3 Marks)
Ans. The molecular weight is the sum of the atomic masses of all the elements present in a molecule. And the information regarding which atoms are present and in what number of those atoms are present in any molecule can be known from its molecular formula. So, the molecular weight can be calculated with the knowledge of the molecular formula of the molecule and the atomic masses of the elements present in it.
Ques7. What is the difference between atomic mass and molecular mass? (3 Marks)
Ans. Atomic mass is the sum of the masses of all the atomic units of an atom. It is calculated as the sum of the masses of protons and neutrons in an atom. While the molecular mass is the sum of the atomic masses of all the elements present in a molecule. It is calculated using the knowledge of the atomic masses of individual elements and their molecular formula.
Ques8. Find out the molecular weight of Carbon dioxide? (3 Marks)
Ans. Carbon dioxide (CO2) has one Carbon atom and 2 Oxygen atoms.
We know that, atomic mass of Carbon is 12.011u
Atomic mass of Oxygen is 15.999u
Hence, the molecular weight of CO2 will be,
12.011u + (2 x 15.999)u
= 12.011u + 31.998u
= 44.009 g/mol
| Chapter Related Articles | ||
|---|---|---|
| Solvent | Concentration of Solutions | Solubility |
| Types of Solutions | Le Chatelier’s Principle | Raoult’s law |
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