his/her chair
Tom se cayó hacia atrás sobre su silla.
Tom fell backwards on his chair.
Mary se desplomó sobre su silla.
Mary slumped into her chair.
Puesto que usted no puede hacer nada aparte de conducir su silla cerca de ella.
Since you can't do anything other than drive your chair near her.
— Intocable (2011)
Ella cedió su silla a una persona mayor.
She gave up her chair to an elderly person.