A 0.1 M solution of which of the following is colorless? — Solutions Chemistry Question

STEPS:

1. Analyze the Ions in Each Solution: Identify the metal cations and polyatomic anions present in the given choices. The color of an aqueous solution is typically determined by the electronic structure of its ions, particularly transition metal ions.
2. Recall the Concept of Color in Solutions: Most transition metal ions have partially filled $d$-orbitals. When these ions form complexes with water molecules, the $d$-orbitals split in energy. Electrons can then absorb specific wavelengths of visible light to jump between these $d$-orbitals (a $d$-$d$ transition), which results in a colored solution.
3. Identify Representative (Main Group) Elements: Ions from Group 1 (alkali metals) and Group 2 (alkaline earth metals) have noble gas electron configurations. They lack partially filled $d$-orbitals and do not undergo electronic transitions that absorb visible light.
4. Evaluate Choice A ($MgCl_2$): Magnesium is a Group 2 element. The $Mg^{2+}$ ion has an electron configuration of $1s^2 2s^2 2p^6$, which is stable and lacks any $d$-electrons. Because it cannot undergo $d$-$d$ transitions, its aqueous solutions are colorless.
5. Conclusion: Since $Mg^{2+}$ is the only ion among the choices that is a representative metal without $d$-orbital transitions, $MgCl_2$ is the correct answer.

WHY_OTHERS_WRONG:

• $Ni(NO_3)_2$ (B): Contains the $Ni^{2+}$ transition metal ion. Nickel(II) ions in water typically form a green solution due to $d$-orbital electronic transitions.
• $Na_2CrO_4$ (C): Contains the chromate ($CrO_4^{2-}$) polyatomic ion. Chromate is a well-known transition metal oxyanion that produces a bright yellow color in solution.
• $KMnO_4$ (D): Contains the permanganate ($MnO_4^-$) polyatomic ion. Even though $Mn$ is in its $+7$ oxidation state (no $d$-electrons), the intense deep purple color arises from a "charge-transfer" transition between oxygen and manganese.
• $CuSO_4$ (E): Contains the $Cu^{2+}$ transition metal ion. In aqueous solution, copper(II) ions coordinate with water to produce a characteristic blue color.