On heating $KMnO_4$,one among the following is not formed:

Cigarette or gas lighter is made up of

The oxidation number of $Mn$ in the product of alkaline oxidative fusion of $MnO_2$ is

Fill in the blanks:
$(1)$ In the dichromate ion,the $O-Cr-O$ bond angle is ....... $^o$.
$(2)$ The reaction $3Sn^{2+} \to 3Sn^{4+} + 6e^-$ occurs via .........,in which $Sn^{2+}$ is ......... .
$(3)$ The symbols for stannous and stannic are ......... and ......... respectively.
$(4)$ $V_2O_4$ dissolves in acidic compounds to give ......... salts.

Select the salt whose aqueous solution is not green.

Which of the following is used in the manufacture of high refractive index optical glass for cameras?