Which among the following compounds in crystalline form is used for making Nicol's prism?

Two electrolytic cells are connected in series containing $\mathrm{CuSO}_4$ solution and molten $\mathrm{AlCl}_3$. If in electrolysis 0.4 moles of 'Cu' are deposited on cathode of first cell. The number of moles of 'Al' deposited on cathode of the second cell is

Mandelonitrile is obtained by the reaction between hydrogen cyanide and

The ionic charges on chromate ion and dichromate ion respectively is