Molar mass is the mass of one mole of a substance β€” it bridges the atomic world (amu) and the lab world (grams). Every stoichiometry calculation starts here.

What Is Molar Mass?

Molar mass is numerically equal to the atomic mass of an element or the sum of atomic masses in a compound, expressed in grams per mole (g/mol).

The key: the atomic mass on the periodic table (in amu) equals the molar mass (in g/mol).

  • Carbon (C): atomic mass = 12.011 amu β†’ molar mass = 12.011 g/mol
  • Oxygen (O): 15.999 g/mol
  • Hydrogen (H): 1.008 g/mol
  • Nitrogen (N): 14.007 g/mol
  • Sodium (Na): 22.990 g/mol
  • Chlorine (Cl): 35.450 g/mol

The Formula

Molar Mass = Ξ£ (Number of each atom Γ— Atomic mass of that element)

Step-by-Step Examples

Example 1: Water (Hβ‚‚O)

  1. Identify atoms: 2 Γ— H, 1 Γ— O
  2. Find atomic masses: H = 1.008, O = 15.999
  3. Calculate: (2 Γ— 1.008) + (1 Γ— 15.999)
  4. = 2.016 + 15.999 = 18.015 g/mol

Example 2: Carbon Dioxide (COβ‚‚)

  1. Atoms: 1 Γ— C, 2 Γ— O
  2. Masses: C = 12.011, O = 15.999
  3. = 12.011 + (2 Γ— 15.999) = 12.011 + 31.998 = 44.009 g/mol

Example 3: Glucose (C₆H₁₂O₆)

  1. Atoms: 6 Γ— C, 12 Γ— H, 6 Γ— O
  2. = (6 Γ— 12.011) + (12 Γ— 1.008) + (6 Γ— 15.999)
  3. = 72.066 + 12.096 + 95.994 = 180.156 g/mol

Example 4: Sodium Chloride (NaCl)

  1. Atoms: 1 Γ— Na, 1 Γ— Cl
  2. = 22.990 + 35.450 = 58.440 g/mol

Example 5: Sulfuric Acid (Hβ‚‚SOβ‚„)

  1. Atoms: 2 Γ— H, 1 Γ— S, 4 Γ— O
  2. S atomic mass = 32.065
  3. = (2 Γ— 1.008) + 32.065 + (4 Γ— 15.999)
  4. = 2.016 + 32.065 + 63.996 = 98.077 g/mol

Handling Parentheses in Formulas

When a formula contains parentheses with a subscript, multiply the subscript by every atom inside:

Calcium hydroxide: Ca(OH)β‚‚

  • Ca: 1
  • O: 1 Γ— 2 = 2
  • H: 1 Γ— 2 = 2
  • = 40.078 + (2 Γ— 15.999) + (2 Γ— 1.008)
  • = 40.078 + 31.998 + 2.016 = 74.092 g/mol

Aluminium sulfate: Alβ‚‚(SOβ‚„)₃

  • Al: 2
  • S: 1 Γ— 3 = 3
  • O: 4 Γ— 3 = 12
  • = (2 Γ— 26.982) + (3 Γ— 32.065) + (12 Γ— 15.999)
  • = 53.964 + 96.195 + 191.988 = 342.147 g/mol

Converting Grams to Moles and Back

Moles = Mass (g) Γ· Molar Mass (g/mol)
Mass (g) = Moles Γ— Molar Mass (g/mol)

Example: How many moles in 50 g of water?

  • Moles = 50 g Γ· 18.015 g/mol = 2.776 mol

Example: What mass is 0.5 mol of glucose?

  • Mass = 0.5 mol Γ— 180.156 g/mol = 90.078 g

Common Molar Masses to Memorise

CompoundFormulaMolar Mass
WaterHβ‚‚O18.015
Carbon dioxideCOβ‚‚44.009
GlucoseC₆H₁₂O₆180.156
Sodium chlorideNaCl58.440
AmmoniaNH₃17.031
MethaneCHβ‚„16.043
EthanolCβ‚‚Hβ‚…OH46.068
Oxygen gasOβ‚‚31.998

Why Molar Mass Matters

Molar mass is the key conversion in stoichiometry β€” it links the symbolic world (chemical equations with mole ratios) to the physical world (what you weigh on a balance). Every yield calculation, every concentration determination, and every limiting reagent problem requires it.