Which of the following metals will react with water?

Two point charges $-q$ and $+q$ are located at points $(0, 0, -a)$ and $(0, 0, a)$ respectively. The electric potential at a point $(0, 0, z)$,where $z > a$ is

$A$ long solenoid has $500$ turns. When a current of $2$ $A$ is passed through it,the resulting magnetic flux linked with each turn of the solenoid is $4 \times 10^{-3}$ Wb. The self-inductance of the solenoid is ....... $H$.

The compound prepared by a substitution reaction of benzene is

$A$ solution of urea (molar mass $60 \ g \ mol^{-1}$) boils at $100.18 \ ^oC$ at atmospheric pressure. If $K_f$ and $K_b$ for water are $1.86$ and $0.512 \ K \ kg \ mol^{-1}$ respectively,the above solution will freeze at ......... $^oC$.

For preparing a $0.1 \, N$ solution of a compound from its impure sample,of which the percentage purity is known,the weight of the substance required will be