Which of the following reagents is used in the Gatterman-Koch formylation of arene?

The experimentally determined molar mass of a non-volatile solute,$BaCl_2$,in water by Cottrell's method,is

Which of the following gases present in the air darkens the surface painted with white lead?

Identify the chiral molecule from the following.

Two polaroids are placed in the path of an unpolarized beam of intensity $I_0$ such that no light is emitted from the second polaroid. If a third polaroid,whose polarization axis makes an angle $\theta$ with the polarization axis of the first polaroid,is placed between these polaroids,then the intensity of light emerging from the last polaroid will be

In an emission spectrum,the transitions $n_2 \to n_1, n_3 \to n_1, n_4 \to n_1, n_5 \to n_1$ result in the emission of energies $E_1, E_2, E_3, E_4$ respectively. What is the increasing order of these energies?