Question

At absolute zero temperature:
a.Only para hydrogen exists
b.Only ortho hydrogen exists
c.Both para and ortho hydrogen exists
d.None of these

Answer 1

Molecular hydrogen or ${H_2}$ consists of two magnetic forms – Para and Ortho. When two hydrogen nuclei are spinning in opposite directions so that their total angular momentum is zero is known as Para hydrogen; and on the other hand, when the hydrogen nuclei are spinning in the same direction, it is known as ortho hydrogen.

Complete answer:
At ordinary temperature, the Dihydrogen or Hydrogen molecule (${H_2}$) contains 75% Ortho and 25% Para hydrogen. When the temperature decreases, the percentage of ortho hydrogen decreases, whereas the percentage of para hydrogen increases.
So at $0K$or absolute zero temperature, only para-hydrogen exists.

Hence, the correct answer is (a) Only para hydrogen exists.

Note:
Para hydrogen has antiparallel nuclear spins $\left( { \downarrow \uparrow } \right)$, having even rotational quantum numbers $(J = 0,2,4,...)$ and is a singlet state. Para and ortho hydrogen was postulated by Heisenberg in 1926 and their existence was confirmed by Bonhoeffer and Harteck in 1929.
When the temperature decreases, ortho hydrogen converts into para hydrogen. As the temperature decreases, the proportion of $p - {H_2}$ increases until it reaches absolute temperature.
The equilibrium state of ortho and para hydrogen is:
${H_2}$(ortho)$= {H_2}$(para)