The aqueous solution of which of the following ion is colourless?

Which pair of metal ions in their respective oxidation states forms colourless compounds?

Which of the following ions are paramagnetic and coloured?

For the transition elements $Cr, Mn, Fe,$ and $Co$,the correct order of the negative values of $E^o_{M^{2+}/M}$ is:

Which of the following series of transition metal ions has a $3d^2$ electronic configuration?

Why is the $E^{\Theta }$ value for the $Mn^{3+}/Mn^{2+}$ couple much more positive than that for $Cr^{3+}/Cr^{2+}$ or $Fe^{3+}/Fe^{2+}$? Explain.