KMnO₄ can be prepared from K₂MnO₄ as per reaction,

 $3{\mathrm{MnO}}_4^{2-} + 2{\mathrm{H}}_2\mathrm{O} \rightleftharpoons 2{\mathrm{MnO}}_4^{-} + {\mathrm{MnO}}_2 + 4{\mathrm{OH}}^{-}$

The reaction can go to completion by removing OH^- ions by adding             

1. HCl

2. KOH

3. CO₂

4. SO₂

10 gm CaCO₃ is taken in a one litre container. The active mass of CaCO₃ is: 

1. 0.1

2. 1

3. zero

4. 10