Which one of the following has different number of molecules? (All kept at normal temperature and pressure)

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NDA (Held On: 18 Sept 2016) General Ability Test Previous Year paper
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  1. 3 gram of Hydrogen 
  2. 48 gram of Oxygen
  3. 42 gram of Nitrogen 
  4. 2 gram of carbon 

Answer (Detailed Solution Below)

Option 4 : 2 gram of carbon 
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Detailed Solution

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The correct answer is option 4 i.e. 2 grams of carbon.

Number of moles

  • The number of moles can be calculated by Mass (g) divided by Molecular mass.
  • \(Number\space of\space moles = {Mass(g) \over Molecular\space mass\space (M)}\)
  • We know the same number of moles have the same number of molecules or atoms.
  • 3 grams of Hydrogen = 1.5 moles, because the molecular mass of Hydrogen is 2.
  • 48 grams of Oxygen= 1.5 moles, because the molecular  mass of Oxygen is 32.
  • 42 grams of Nitrogen = 1.5 moles, because the molecular mass of Nitrogen is 28.
  • 2 grams of carbon = 0.167 moles, because the molecular mass of carbon is 14.
  • Hence option 4 is correct.
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