Answer: 100% para-H 2 (para is lower energy at low T).
- A 100% para-H<sub>2</sub> (para is lower energy at low T)
- B 100% ortho-H<sub>2</sub>
- C 50:50
- D 75:25 ortho:para (room temperature equilibrium)
Correct answer: A. 100% para-H<sub>2</sub> (para is lower energy at low T)
Explanation: para-H<sub>2</sub> (antiparallel nuclear spins) is the lower energy ground state; at near 0 K, essentially all H<sub>2</sub> is para form.
Each water molecule's O-H bonds (covalent, solid) can hydrogen-bond (dashed) to neighbouring molecules, forming an extended network responsible for water's unusually high boiling point and density anomaly.
Concept context
The simplest and most abundant element in the universe. Study its unique position in the periodic table, isotopes (protium, deuterium, tritium), properties of water and hydrogen peroxide, and industrial uses.