Question

Sol. [Key Concept: Comparing numbers of helium atoms]

Helium is monatomic. Using the stated approximate atomic mass, 4 g corresponds to about 1 mol or Nₐ atoms, 2.271098 L at 273.15 K and 1 bar corresponds to about 0.1 mol, and 4 u corresponds to approximately one atom. Four moles contain 4Nₐ atoms, the largest number. Hence, option (3). Using the older 1 atm convention for STP does not change the answer.

1- 4 g of Helium $= \frac{4\text{ }g}{4\text{ }g}$ mole $= 1$ mole $= N_{A}$ He atom

2- 2.2710982 of He at STP $= \frac{2.271}{22.710982}$ mole

$$\begin{matrix} & \ = 0.1\text{ }mole \\ & \ = 0.1{\text{ }N}_{A}He\text{~atom~} \end{matrix}$$

3- 4 mol of $He = 4{\text{ }N}_{A}He$ atoms

4- $4u$ of $He = \frac{4u}{4u} = 1He$ atom