The molecular weight of a substance is the sum of the atomic weights of all the atoms in the substance's molecule. It can be calculated by adding the atomic weights of all the atoms in the molecule's chemical formula, or it can be found using a periodic table or an atomic weight database. In this article, we will study in detail how to find molecular weight of a substance.
Molecular Weight- Introduction
Molecular weight is an important concept in chemistry and is used to determine the size and mass of a molecule. A substance's molecular weight is the sum of the atomic weights of all the atoms in a molecule of the substance. This value is essential in determining the physical and chemical properties of a substance, such as its boiling point, density, and solubility. A substance's molecular weight can be calculated by adding up the atomic weights of all the atoms present in the chemical formula of the molecule. For example, the molecular weight of water (H2O) is calculated by adding up the atomic weight of two hydrogen atoms (1.01 g/mol) and one oxygen atom (16.00 g/mol), giving a total molecular weight of 18.02 g/mol.
1. How can Molecular Weight be Determined?
A substance's molecular weight can be determined using several methods including
Mass Spectrometry: The mass of individual molecules can be determined using a mass spectrometer, which ionizes the sample and sorts the ions based on their mass-to-charge ratio.
Gel Permeation Chromatography: This method uses a column packed with a porous material to separate molecules based on their size. The molecular weight can be calculated from the elation time of the sample.
Ultraviolet-Visible Spectroscopy: The molecular weight can be determined by measuring the extent of absorption or scattering of ultraviolet or visible light by a sample.
Light Scattering: Light scattering techniques, such as dynamic light scattering, can be used to determine the molecular weight of a sample based on the size of the scattered light.
Isotopic Labeling: Isotopic labeling involves replacing one or more of the atoms in a molecule with an isotope and then measuring the mass of the labeled molecule. The molecular weight can be determined from the mass difference between the labeled and unlabeled molecules. Another way to find the molecular weight of a substance is to use a periodic table or a database of atomic weights. These resources provide the atomic weights of all known elements, making it easy to determine the molecular weight of any substance by simply adding up the weights of its constituent atoms.
Another way to find the molecular weight of a substance is to use a periodic table or a database of atomic weights. These resources provide the atomic weights of all known elements, making it easy to determine the molecular weight of any substance by simply adding up the weights of its constituent atoms.
2. How is Molecular Weight Expressed?
A substance's molecular weight is expressed in atomic mass units (amu) or grams per mole (g/mol), which is equivalent to the mass of a single proton or neutron. It can also be expressed in kilograms (kg), grams (g), or pounds (lb). In the field of biochemistry, the term "molecular weight" is often used interchangeably with "mass" or "mass of a substance," referring to the total mass of all atoms in a molecule or the sum of the atomic masses of the individual atoms composing a molecule. Also Read: What is Atomic Mass?
3. Importance of Molecular Weight
A substance's molecular weight is an important characteristic that provides information about its size and behavior in various applications, such as:
Biochemistry and Biology: The molecular weight is crucial in the study of biochemical and biological systems as it affects the solubility, stability, and reactivity of molecules.
Pharmaceuticals: The molecular weight of a drug molecule influences its pharmacokinetics, including its absorption, distribution, metabolism, and excretion.
Polymer Science: The molecular weight of a polymer determines its physical properties, such as its melting temperature, viscosity, and elasticity.
Environmental Science: The molecular weight is a key factor in the behavior of contaminants in the environment and helps determine their fate and transport.
Materials Science: The molecular weight of a material can influence its mechanical properties and processing behavior, such as its strength, toughness, and flow behavior.
4. Sample Calculations Q.
Calculate the molecular weight of the following: (i) H2SO4 (ii) NH3 (iii) CaCO3 (iv) C2H5OH (v) NaHCO3 Solution: (i) H2SO4 2 × (1) + 1 × (32) + 4 × (16) The molecular Weight of H2SO4 is 98 g/mol. (ii) NH3 1 × (14) + 3 × (1) The molecular weight of NH3 is 17 g/mol. (iii) CaCO3 1 × (40) + 1 × (12) + 3 × (16) The molecular weight of CaCO3 is 100 g/mol. (iv) C2 H5 OH 2 × (12) + 6 × (1) + 1 × (16) The molecular weight of C2H5OH is 46 g/mol. (v) NaHCO3 1 × (23) + 1 × (1) + 1 × (12) + 3 × (16) The molecular weight of NaHCO3 is 84 g/mol.
Key Takeaways
Molecular weight is a crucial concept in chemistry and plays a significant role in determining the size and mass of a molecule.
Molecular weight can be calculated by adding up the atomic weights of all the atoms present in a molecule or by using a periodic table or a database of atomic weights.
Understanding molecular weight is important in many scientific and industrial applications, and its importance cannot be overstated.
FAQs
1. What is molecular weight?
Molecular weight is the sum of the atomic weights of all the atoms in a molecule. It is expressed in atomic mass units (amu).
2. How do you calculate molecular weight?
The molecular weight can be calculated by adding up the atomic weights of all the atoms in a molecule. The atomic weight of each element can be found on the periodic table.
3. How do you find the molecular weight of a compound?
To find the molecular weight of a compound, add up the atomic weights of all the atoms in the compound's chemical formula.
4. What is the formula for calculating molecular weight?
The formula for calculating molecular weight is: MW = Σ (atomic weight of each element in the molecule)
5. How does molecular weight affect properties of a substance?
Molecular weight affects properties of a substance, such as boiling point, surface tension, and viscosity, because it influences the strength of intermolecular forces and the mass of particles in a substance.
Conclusion
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