The species having identical radii according to the Bohr's theory are :

A. H (first orbit)

B. $\mathrm{He}^{+}$(first orbit)

C. $\mathrm{He}^{+}$(Second orbit)

D. $\mathrm{Li}^{2+}$ (first orbit)

E. $\mathrm{Be}^{3+}$ (Second orbit)

Choose the correct answer from the options given below :

Which of the following pictorial diagram most correctly represents the $\pi^*$ ( $\pi$ antibonding) molecular orbital between two atoms if the internuclear axis is taken to be in the $z$-direction $(\xrightarrow{z \text {-axis }})$ ?

At $27^{\circ} \mathrm{C}, 0.1 \mathrm{M}, 1 \mathrm{~L} \mathrm{~K}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]$ aqueous solution and $0.1 \mathrm{M}, 1 \mathrm{~L} \mathrm{FeCl}_3$ aqueous solution are placed in a container separated by a semi permeable membrane AB . Assume complete dissociation of both the solutes. Which of the following statement is correct?

20 mL of a solution of acetic acid required 28.4 mL of 0.1 M NaOH for its neutralization. A solution (X) was prepared by mixing 20 mL of the above acetic acid and 14.2 mL of 0.1 M NaOH solution. What is the pH of the solution $(\mathrm{X})$ ? $\left(\mathrm{pK}_{\mathrm{a}}\right.$ value of acetic acid is 4.75).