Final answer:
The equilibrium bond length of HCl in its ground state is 127.5 pm. This value represents the molecule's most stable state at its lowest energy level.
Step-by-step explanation:
The equilibrium bond length of HCl when the molecule is in its ground state is 127.5 picometers (pm). This measurement is determined experimentally using methods such as infrared spectroscopy, which analyses the vibrational energies of the molecule. The ground state refers to the state of the molecule where it is at its lowest energy level. The HCl molecule consists of a hydrogen atom and a chlorine atom. Due to the significant mass difference between the two atoms, we can approximate the vibrations of HCl by assuming the Cl atom is stationary and the H atom exhibits harmonic oscillations. The equilibrium bond length corresponds to the separation distance where the molecule's potential energy is at a minimum.