The radius of which of the following orbits is the same as that of the $1^{st}$ orbit of $H$ atom?

Bamboo and grasses elongate by the activity of ......

The normal freezing point $(f.p.)$ of nitrobenzene is $278.82\, K$. If a non-volatile solute is dissolved in it and a solution of $0.25\, m$ (molality) is prepared,the observed $f.p.$ of the solution is $276.82\, K$. Evaluate $K_f$ of nitrobenzene in $K\, kg\, mol^{-1}$.

In space, the equation $by + cz + d = 0$ represents a plane perpendicular to the

Consider the following statements $A$ and $B$ and identify the correct answer:
$A$. Fresnel's diffraction pattern occurs when the source of light or the screen on which the diffraction pattern is seen,or when both,are at a finite distance from the aperture.
$B$. Diffracted light can be used to estimate the helical structure of nucleic acids.

$20 \text{ mL}$ of $0.1 \text{ M}$ acetic acid is mixed with $50 \text{ mL}$ of potassium acetate. $K_a$ of acetic acid $= 1.8 \times 10^{-5}$ at $27^\circ \text{C}$. Calculate the concentration of potassium acetate if the $pH$ of the mixture is $4.8$. (in $\text{ M}$)