How much heat is needed to vaporise 9.0 g of water at its boiling point (ΔHvap = 40.7 kJ mol⁻¹, H₂O = 18 g mol⁻¹)?
Correct answer D. 20.4 kJ
Explanation
Moles of water = 9.0 ÷ 18 = 0.50 mol. Heat = 0.50 × 40.7 kJ mol⁻¹ = 20.4 kJ. Multiplying by mass instead of moles gives 366 kJ.
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