Molar Mass Calculations

Molar Mass: Explained Notes for Students

What is Molar Mass?

• Molar mass is the mass of one mole of any substance.
• It is expressed in grams per mole (g/mol).
• It tells us how much one mole of atoms, molecules, or formula units of that substance weighs.
• Every element has its own atomic mass, which can be found on the periodic table.
• The molar mass of a compound is the total mass of all the atoms present in one mole of that compound.

How to Calculate Molar Mass (Step-by-Step)

1. Write down the chemical formula of the element or compound.
2. Count the atoms of each element in the formula.
3. Look up the atomic mass (in g/mol) of each element from the periodic table.
4. Multiply the atomic mass by the number of atoms of that element.
5. Add up all the masses to get the total molar mass of the compound.

Examples

Example 1: Silver (Ag)

• Formula: Ag
• Atoms: 1 Ag atom
• Atomic mass of Ag = 107.87 g/mol
• Calculation: 1 × 107.87 = 107.87 g/mol
• Molar mass = 107.87 g/mol

Example 2: Chlorine (Cl)

• Formula: Cl
• Atoms: 1 Cl atom
• Atomic mass of Cl = 35.45 g/mol
• Calculation: 1 × 35.45 = 35.45 g/mol
• Molar mass = 35.45 g/mol

Example 3: Sodium Sulfate $(Na_2SO_4)$

• Formula: $(Na_2SO_4)$
• Atoms: 2 Na, 1 S, 4 O
• Atomic masses: Na = 22.99, S = 32.07, O = 16.00 g/mol
• Calculation:
  $2 \times 22.99 = 45.98$
  $1 \times 32.07 = 32.07$
  $4 \times 16.00 = 64.00$
• Total molar mass = 45.98 + 32.07 + 64.00 = 142.05 g/mol

Example 4: Urea $(CO(NH_2)_2)$

• Formula: $(CO(NH_2)_2)$
• Atoms: 1 C, 1 O, 2 N, 4 H
• Atomic masses: C = 12.01, O = 16.00, N = 14.01, H = 1.008 g/mol
• Calculation:
  $1 \times 12.01 = 12.01$
  $1 \times 16.00 = 16.00$
  $2 \times 14.01 = 28.02$
  $4 \times 1.008 = 4.03$
• Total molar mass = 12.01 + 16.00 + 28.02 + 4.03 = 60.06 g/mol

Summary Tips for Students

• Always start with the chemical formula.
• Be careful to multiply correctly when there are subscripts.
• Use the latest periodic table for atomic masses.
• Add all the partial masses to get the final molar mass.
• Practice with different formulas to become confident.

For more information visit: Mole Concept