Question: 50. The highest number of helium atoms is in :
(1) 4 mol of helium
(2) 4 u of helium
(3) 4 g of helium
(4) 2.271098 L of helium at STP
Answer: Option (1)
Explanation:
The number of atoms depends on the number of moles present.
For option (1), 4 mol of helium contains 4 \times N_A atoms,
where N_A is Avogadro’s number.
For option (2), the atomic mass of helium is 4 u,
so 4 u of helium corresponds to the mass of a single helium atom,
which contains only one atom.
For option (3), the molar mass of helium is 4 \mathrm{~g~mol}^{-1},
so 4 g of helium corresponds to 1 mol, which contains N_A atoms.
For option (4), at STP, 1 mol of any gas occupies 22.71 \mathrm{~L}.
Thus, 2.271098 \mathrm{~L} of helium corresponds to approximately
\frac{2.271098}{22.71} = 0.1 mol, which contains 0.1 N_A atoms.
Comparing all options, 4 mol of helium contains the highest number of atoms.
Therefore, the correct answer is Option (1).