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What Is Molecular Weight?
Molecular weight (or molar mass) is the sum of the atomic weights of all atoms in a molecule, expressed in grams per mole (g/mol). For example, water (H2O) has a molecular weight of 18.015 g/mol: two hydrogen atoms (2 x 1.008) plus one oxygen atom (15.999). Molecular weight is essential for converting between mass and moles in chemistry.
Using Molecular Weight in Calculations
To find the number of moles in a given mass: moles = mass (g) / molecular weight (g/mol). To find the number of molecules: multiply moles by Avogadro's number (6.022 x 10^23). Molecular weight is used in stoichiometry, solution preparation, dosing calculations, and chemical analysis.
Frequently Asked Questions
Molecular weight refers to covalent molecules (like H2O or CO2). Formula weight is used for ionic compounds (like NaCl) that do not exist as discrete molecules. Both are calculated the same way by summing atomic weights, but the terminology differs.
A mole is 6.022 x 10^23 particles (Avogadro's number). One mole of any substance contains this many molecules or formula units. One mole of water (18.015 g) contains 6.022 x 10^23 water molecules.
Molecular weight allows chemists to weigh out specific numbers of molecules for reactions. It connects the macroscopic world (grams on a balance) to the microscopic world (individual molecules). Correct stoichiometric calculations depend on accurate molecular weights.