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How to determine the number of moles (of molecules or formula units) in each sample:

a) 25.5 g NO\(_2\)

b) 1.25 kg CO\(_2\)

c) 38.2 g KNO\(_3\)

Answer :

25.5 g NO₂, 1.25 kg CO₂, and 38.2 g KNO₃ are provided, yielding 0.554, 28.40, and 0.378 moles respectively.

Number of moles = Mass of the sample (g) / Molar mass of the compound (g/mol)

a) 25.5 g NO₂

  1. Molar mass of NO₂ = 14.01 g/mol (N) + 2 × 16.00 g/mol (O) = 46.01 g/mol
  2. Number of moles = 25.5 g / 46.01 g/mol ≈ 0.554 moles

b) 1.25 kg CO₂

  1. Convert kg to g: 1.25 kg = 1250 g
  2. Molar mass of CO₂ = 12.01 g/mol (C) + 2 × 16.00 g/mol (O) = 44.01 g/mol
  3. Number of moles = 1250 g / 44.01 g/mol ≈ 28.40 moles

c) 38.2 g KNO₃

  1. Molar mass of KNO₃ = 39.10 g/mol (K) + 14.01 g/mol (N) + 3 × 16.00 g/mol (O) = 101.11 g/mol
  2. Number of moles = 38.2 g / 101.11 g/mol ≈ 0.378 moles

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