How many moles are in 36 g of water (H₂O, M=18)?

**The reasoning** The formula connecting mass, moles, and molar mass is: **Number of moles (n) = Mass (m) / Molar mass (M)** You're given: - Mass of water = 36 g - Molar mass of H₂O = 18 g/mol Substituting into the formula: n = 36 g ÷ 18 g/mol = **2 moles** This principle is called the **mole concept** — it's the bridge between the mass you can weigh and the number of particles (atoms/molecules) you're working with. **Why the wrong options tempt you** - **Option A (0.5)**: You might flip the formula and do 18÷36 instead of 36÷18. Always keep "what you have ÷ what one mole weighs." - **Option B (1)**: Possibly confusing the molar mass (18) with the answer, or miscalculating. - **Option D (4)**: Maybe doubling 2 accidentally, or multiplying instead of dividing. **Quick takeaway** "To find moles, always divide the mass you have by the molar mass — think of it as 'how many 18 g portions fit into 36 g?'"