Iron is rendered passive by treatment with

Identify the correct order of oxidizing power.

Iron may be rendered passive by treatment with a concentrated solution of

Identify the correct statement about properties of interstitial compounds.

The ${E^0}_{M^{3+}/M^{2+}}$ values for $Cr, Mn, Fe$ and $Co$ are $-0.41, +1.57, +0.77$ and $+1.97 \ V$ respectively. For which one of these metals is the change in oxidation state from $+2$ to $+3$ the easiest?

How many ions liberate $H_2$ gas with dilute acids among the given species:
$Mn^{2+}, Cr^{3+}, Cr^{2+}, Zn^{2+}, Cu^{2+}, Sc^{3+}, Hg^{2+}$