Which transition element of the $3d$ series is used in a dry battery cell?

Thermal decomposition of a $Mn$ compound $(X)$ at $513 \ K$ results in compound $Y$,$MnO_2$ and a gaseous product. $MnO_2$ reacts with $NaCl$ and concentrated $H_2SO_4$ to give a pungent gas $Z$. Identify $X, Y$ and $Z$.

Name the oxometal anions of the first series of the transition metals in which the metal exhibits the oxidation state equal to its group number.

Which of the following statements is incorrect?

Which compounds are formed when $Mn_2O_7$ and $CrO_3$ react with water,given that they are acidic in nature?

$Fe_2Cr_2O_4 + Na_2CO_3 + O_2$ $\xrightarrow{\text{Fusion}} [X]$ $\xrightarrow[H_2O]{H^{+}} [Y]$ $\xrightarrow{H_2O_2/H^{+}} [Z]$
Which of the following statements is true for the compounds $[X]$,$[Y]$,and $[Z]$?