JEE Main 2023 · 11 April · evening · Numerical answer · +4 / −0
Mg(NO₃)₂ · XH₂ O and Ba(NO₃)₂ · YH₂ O, represent formula of the crystalline forms of nitrate salts. Sum of X and Y is _____________.
Answer and solution
Answer 6
Solution
Chemistry ma’am · 4 steps
The smaller the ion, the more water it holds on to: Mg²⁺ keeps six, Ba²⁺ keeps none.
- Size decides hydration. Going down group 2, the ions get bigger, so their charge density and hydration enthalpy fall. A small, highly charged ion binds water strongly and carries it into the crystal.
- Magnesium nitrate. Mg²⁺ is the smallest ion in the group and is strongly hydrated. Its nitrate crystallises from water as the hexahydrate Mg(NO₃)₂.6H₂O, so X = 6.
- Barium nitrate. Ba²⁺ is a large ion with a much lower hydration enthalpy. Its nitrate crystallises as the anhydrous salt Ba(NO₃)₂, so Y = 0.
X + Y = 6 + 0
- So. The sum of X and Y is 6.
The common slip: 8. Treating barium nitrate like BaCl₂.2H₂O and taking Y = 2 gives 8. Barium nitrate crystallises anhydrous, so Y = 0.
The idea to keep. Down group 2, water of crystallisation falls as the ion grows: Mg salts are heavily hydrated, Ba salts often crystallise with little or no water.
Answer from the NTA final answer key. Solution by PrepWiser.