A cubical box of side 1 m contains helium gas (atomic weight 4) at a pressure of   ${\text{100 Nm}}^{–2}$. During an observation time of 1 second, an atom travelling with root- mean-square speed parallel to one of the edges of the cube was found to make  500 hits with a particular wall, without any collision with other atoms. Take R =  $\frac{25}{3}$  ${\text{J mol}}^{–\text{1}}{\text{K}}^{–\text{1}}$  and k = $\text{1}.\text{38}\times \text{1}{0}^{–\text{23}}{\text{J K}}^{–\text{1}}$. Evaluate average kinetic energy.

• A

  $\text{3}.\text{312}\times \text{1}{0}^{–\text{21}}\text{J}$

• B

  $\text{6}.4\text{12}\times \text{1}{0}^{–\text{21}}\text{J}$

• C

  $\text{6}.4\text{12}\times \text{1}{0}^{–\text{20}}\text{J}$

• D

  $\text{8}.143\times \text{1}{0}^{–\text{20}}\text{J}$