While preparing crystals of Mohr's salt, dil $$\mathrm{H}_2 \mathrm{SO}_4$$ is added to a mixture of ferrous sulphate and ammonium sulphate, before dissolving this mixture in water, dil $$\mathrm{H_2SO_4}$$ is added here to :

The number of ions from the following that have the ability to liberate hydrogen from a dilute acid is _________.

$$\mathrm{Ti}^{2+}, \mathrm{Cr}^{2+} \text { and } \mathrm{V}^{2+}$$

The metal that shows highest and maximum number of oxidation state is :

The element which shows only one oxidation state other than its elemental form is :