Normal molar mass is calculated by summing the atomic masses of all the atoms present in one molecule of a substance. The atomic masses are found on the periodic table, typically expressed in atomic mass units (amu) or grams per mole (g/mol).
Calculating Molar Mass: A Step-by-Step Guide
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Identify the chemical formula: Determine the precise chemical formula of the compound you're working with. For example, the formula for water is H₂O, and for ammonia is NH₃.
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Find atomic masses: Locate each element in the periodic table and note its atomic mass. These values are usually given to several decimal places. For instance, the atomic mass of hydrogen (H) is approximately 1.01 g/mol, and oxygen (O) is approximately 16.00 g/mol.
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Multiply by subscripts: Multiply each element's atomic mass by its subscript in the chemical formula. In H₂O, there are two hydrogen atoms, so we multiply 1.01 g/mol by 2. Oxygen has a subscript of 1, so its atomic mass remains 16.00 g/mol.
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Sum the masses: Add the results from step 3 to obtain the total molar mass. For water (H₂O): (2 1.01 g/mol) + (1 16.00 g/mol) = 18.02 g/mol. For ammonia (NH₃): (1 14.01 g/mol) + (3 1.01 g/mol) = 17.04 g/mol.
Examples
- Water (H₂O): (2 × 1.008 amu) + (1 × 15.999 amu) ≈ 18.015 amu or g/mol
- Carbon Dioxide (CO₂): (1 × 12.011 amu) + (2 × 15.999 amu) ≈ 44.01 amu or g/mol
- Glucose (C₆H₁₂O₆): (6 × 12.011 amu) + (12 × 1.008 amu) + (6 × 15.999 amu) ≈ 180.156 amu or g/mol
This method applies to simple molecules. For polymers or substances with varying molecular weights, average molar mass calculations using techniques like number-average molecular weight or weight-average molecular weight are employed https://www.agilent.com/cs/library/technicaloverviews/public/5990-7890EN.pdf. The concept of normality, as mentioned in some references https://www.sigmaaldrich.com/US/en/support/calculators-and-apps/molarity-calculator, relates to molarity but accounts for the number of equivalents per mole of a substance in reactions involving acid-base or redox titrations.
