Which of the following is the weakest base?

The equivalent weight of $H_3PO_4$ in the following reaction is:
$H_3PO_4 + Ca(OH)_2 \to CaHPO_4 + 2H_2O$

Energy per unit volume for a capacitor having area $A$ and separation $d$ kept at potential difference $V$ is given by

The number of possible enantiomeric pairs that can be produced during mono-chlorination of $2$-methylbutane is

During sodium nitroprusside test of sulphide ion in an aqueous solution, one of the ligands coordinated to the metal ion is converted to

In a compound microscope,the focal lengths of two lenses are $1.5 \ cm$ and $6.25 \ cm$. An object is placed at $2 \ cm$ from the objective lens and the final image is formed at $25 \ cm$ from the eye lens. The distance between the two lenses is $.... \ cm$.