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How to Calculate Molar Mass in a Chemical Formula

Published in Chemistry Calculations 2 mins read

The molar mass of a compound is calculated by summing the atomic masses of all the atoms present in its chemical formula. Here's a step-by-step guide:

  1. Identify the Chemical Formula: Know the chemical formula of the compound. For example, water is H₂O, and glucose is C₆H₁₂O₆.

  2. Determine the Number of Atoms of Each Element: Break down the formula to identify each element and the number of atoms of each.

    • For H₂O: 2 Hydrogen (H) atoms and 1 Oxygen (O) atom.
    • For C₆H₁₂O₆: 6 Carbon (C) atoms, 12 Hydrogen (H) atoms, and 6 Oxygen (O) atoms.

  3. Find the Atomic Mass of Each Element: Look up the atomic mass of each element on the periodic table. Atomic masses are typically found beneath the element's symbol.

    • Hydrogen (H) ≈ 1.01 atomic mass units (amu)
    • Carbon (C) ≈ 12.01 amu
    • Oxygen (O) ≈ 16.00 amu

  4. Multiply Atomic Mass by the Number of Atoms: For each element, multiply its atomic mass by the number of atoms of that element in the compound.

    • For H₂O:
      • Hydrogen: 2 atoms * 1.01 amu/atom = 2.02 amu
      • Oxygen: 1 atom * 16.00 amu/atom = 16.00 amu
    • For C₆H₁₂O₆:
      • Carbon: 6 atoms * 12.01 amu/atom = 72.06 amu
      • Hydrogen: 12 atoms * 1.01 amu/atom = 12.12 amu
      • Oxygen: 6 atoms * 16.00 amu/atom = 96.00 amu

  5. Sum the Masses: Add up the masses calculated for each element to get the molar mass of the compound.

    • For H₂O: 2.02 amu + 16.00 amu = 18.02 amu
    • For C₆H₁₂O₆: 72.06 amu + 12.12 amu + 96.00 amu = 180.18 amu

  6. Express in Grams Per Mole (g/mol): Replace "amu" with "g/mol" to express the molar mass.

    • Molar mass of H₂O = 18.02 g/mol
    • Molar mass of C₆H₁₂O₆ = 180.18 g/mol

Example:

Calculate the molar mass of sodium chloride (NaCl).

  1. Formula: NaCl
  2. Atoms: 1 Sodium (Na) atom, 1 Chlorine (Cl) atom
  3. Atomic Masses: Na ≈ 22.99 amu, Cl ≈ 35.45 amu
  4. Multiply:
    • Na: 1 * 22.99 amu = 22.99 amu
    • Cl: 1 * 35.45 amu = 35.45 amu
  5. Sum: 22.99 amu + 35.45 amu = 58.44 amu
  6. Molar Mass: 58.44 g/mol

Therefore, the molar mass of NaCl is 58.44 g/mol.

Calculating molar mass is crucial for stoichiometry and converting between mass and moles in chemical reactions.

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