In the coordination compound $K_4[Ni(CN)_4]$,the oxidation state of nickel is:

The temperature of the hot junction of a thermocouple changes from $80\,^{\circ}C$ to $100\,^{\circ}C$. The percentage change in thermoelectric power is ............... $\%$.

$A$ certain electronic transition from an excited state to the ground state of the $H$ atom in one or more steps gives rise to three lines in the ultraviolet region of the spectrum. How many lines does this transition produce in the infrared region of the spectrum?

If the line $y=2x+c$ is a tangent to the circle $x^2+y^2=5$,then a value of $c$ is

In a potentiometer experiment,the balancing length for a cell is $240 \ cm$. When a resistance of $2 \ \Omega$ is connected in parallel with the cell,the balancing length becomes $120 \ cm$. The internal resistance of the cell is ................. $\Omega$.

The oxoanion which contains all equivalent $M-O$ bonds is:
$(I)$ $CrO_4^{2-}$
$(II)$ $MnO_4^{-}$
$(III)$ $Cr_2O_7^{2-}$