$BX_3 + NH_3 \xrightarrow{R.T} BX_3 \cdot NH_3 + \text{Heat of adduct formation } (\Delta H)$. The numerical value of $\Delta H$ is found to be maximum for:

Which of the following pairs of transition metal ions have the same calculated values of magnetic moment?

If $x=\log _{0.1} 0.001$ and $y=\log _9 81$,then $\sqrt{x-2 \sqrt{y}}$ is equal to

The radiation emitted by a star $A$ is $10,000$ times that of the sun. If the surface temperatures of the sun and the star $A$ are $6000 \ K$ and $2000 \ K$ respectively,the ratio of the radii of the star $A$ and the sun is (in $:1$)

$A$ spherical surface of radius of curvature $R$ separates air (refractive index $1.0$) from glass (refractive index $1.5$). The centre of curvature is in the glass. $A$ point object $P$ placed in air is found to have a real image $Q$ in the glass. The line $PQ$ cuts the surface at the point $O$,and $PO = OQ$. The distance $PO$ is equal to: (in $,R$)

If the circle $x^2+y^2+6x-2y+k=0$ bisects the circumference of the circle $x^2+y^2+2x-6y-15=0$,then $k$ is equal to :