This is the 68th most frequent Catalan word.
qual
"Qual" is the Catalan word for "which" (relative pronoun).
'Pel qual' is a combination of 'per' (through) and 'el qual', indicating a path or way related to the verb.
No sabia pel qual camí hauríem d'anar.
I didn't know through which path we should go.
'Per la qual' means 'why' or 'the reason for which', used to explain a reason or cause.
La raó per la qual estic aquí és ajudar-te.
The reason why I am here is to help you.
In this sentence, 'el qual' is used as a relative pronoun to introduce a relative clause referring to a specific antecedent.