Heating phthalic anhydride and concentrated sulfuric acid with which of the following yields phenolphthalein?

$A$ public park,in the form of a square,has an area of $(100 \pm 0.2) \; m^2$. The side of the park is ............ $m$.

Which of the following is a Lewis acid?

Explain the reason for the fusion of an organic compound with metallic sodium for testing nitrogen,sulphur and halogens.

$A$ photoelectric surface is illuminated successively by monochromatic light of wavelengths $\lambda$ and $\frac{\lambda}{2}$. If the maximum kinetic energy of the emitted photoelectrons in the second case is $3$ times that in the first case,the work function of the surface is:

The condition for the coaxial system $x^2+y^2+2\lambda x+c=0$,where $\lambda$ is a parameter and $c$ is a constant,to have distinct limiting points,is