Calculating Molar Mass

Overview

• Molar mass is the mass of a given substance (chemical element or chemical compound) divided by its amount of substance.
• Also known as molecular weight or formula weight.
• Calculated from the chemical formula by adding the atomic masses of all atoms in the molecule.

Process for Calculation

1. Identify Atoms in the Formula

   • Look for each element in the chemical formula.
   • If an element has a subscript, it indicates the number of atoms of that element.

2. Use the Periodic Table

   • Find atomic weight of each element.
   • Use consistent units (grams per mole).

3. Multiply and Sum Weights

   • Multiply the atomic weight by the number of atoms for each element.
   • Add all these values to get the total molar mass.

Examples

Example 1: SO₂

• Chemical Formula: SO₂
  • 1 Sulfur (S)
  • 2 Oxygen (O)
• Calculations:
  • Sulfur: 1 x 32.07 g/mol
  • Oxygen: 2 x 16.00 g/mol
  • Total: 32.07 + 32.00 = 64.07 g/mol
• Result: Molar mass of SO₂ is 64.07 g/mol

Example 2: C₃H₈O

• Chemical Formula: C₃H₈O
  • 3 Carbon (C)
  • 8 Hydrogen (H)
  • 1 Oxygen (O)
• Calculations:
  • Carbon: 3 x 12.01 g/mol
  • Hydrogen: 8 x 1.01 g/mol
  • Oxygen: 1 x 16.00 g/mol
  • Total: 36.03 + 8.08 + 16.00 = 60.11 g/mol
• Result: Molar mass of C₃H₈O is 60.11 g/mol

Example 3: Ca(NO₃)₂

• Chemical Formula: Ca(NO₃)₂
  • 1 Calcium (Ca)
  • 2 Nitrogen (N)
  • 6 Oxygen (O)
• Calculations:
  • Calcium: 1 x 40.08 g/mol
  • Nitrogen: 2 x 14.01 g/mol
  • Oxygen: 6 x 16.00 g/mol
  • Total: 40.08 + 28.02 + 96.00 = 164.10 g/mol
• Result: Molar mass of Ca(NO₃)₂ is 164.10 g/mol

Example 4: (NH₄)₃PO₄

• Chemical Formula: (NH₄)₃PO₄
  • 3 Nitrogen (N)
  • 12 Hydrogen (H)
  • 1 Phosphorus (P)
  • 4 Oxygen (O)
• Calculations:
  • Nitrogen: 3 x 14.01 g/mol
  • Hydrogen: 12 x 1.01 g/mol
  • Phosphorus: 1 x 30.97 g/mol
  • Oxygen: 4 x 16.00 g/mol
  • Total: 42.03 + 12.12 + 30.97 + 64.00 = 149.12 g/mol
• Result: Molar mass of (NH₄)₃PO₄ is 149.12 g/mol

Example 5: MgSO₄·7H₂O (Hydrate)

• Chemical Formula: MgSO₄·7H₂O
  • 1 Magnesium (Mg)
  • 1 Sulfur (S)
  • 4 Oxygen (O) + 7 from H₂O = 11 Oxygen (O)
  • 14 Hydrogen (H)
• Calculations:
  • Magnesium: 1 x 24.31 g/mol
  • Sulfur: 1 x 32.07 g/mol
  • Oxygen: 4 x 16.00 g/mol inside MgSO₄
  • Hydrogen: 14 x 1.01 g/mol
  • Oxygen (water): 7 x 16.00 g/mol
  • Total: 24.31 + 32.07 + 64.00 + 14.14 + 112.00 = 246.46 g/mol
• Result: Molar mass of MgSO₄·7H₂O is 246.46 g/mol

Important Notes

• Be aware of parentheses and dots in formulas indicating multiple groups or hydrates.
• Always check periodic table for the most accurate atomic weights.
• The molar mass is always expressed in grams per mole (g/mol).