Study Guides/Chemistry/Molar Mass of Chlorine
Study Guide · Chemistry

Molar Mass of Chlorine

In chemistry calculations, knowing the molar mass of Chlorine is essential for stoichiometry and gas law problems.

Question (Click to Flip)

How many moles are in 142 g of Cl₂?

Answer

Moles = Mass / Molar mass = 142 / 71 = 2 moles of Cl₂

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Key Facts

Always use 71 g/mol for Cl₂ (gas) in reactions, but 35.5 g/mol for a single Cl atom in ionic calculations.

Molar Mass Values

  • Chlorine Atom (Cl): Molar mass = 35.5 g/mol
    • (Chlorine has two isotopes: Cl-35 and Cl-37 in 3:1 ratio, giving average = 35.5)
  • Chlorine Gas (Cl₂): Molar mass = 2 × 35.5 = 71 g/mol
    • (Chlorine exists as a diatomic molecule in its standard state)

Why 35.5 and not a whole number?

Chlorine has two naturally occurring isotopes:

  • Cl-35: 75% abundance
  • Cl-37: 25% abundance

Weighted average = (35 × 75 + 37 × 25) / 100 = (2625 + 925) / 100 = 35.5 g/mol

Questions and Answers

How many moles are in 142 g of Cl₂?+

Moles = Mass / Molar mass = 142 / 71 = **2 moles** of Cl₂

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