I want to stay online on whatsapp web whenever i want after scanning the qr code but with mobile data being off on mobile. so is it possible? I have search a lot but didn't get a possible solution!! please help if there is!!
Simply it is not possible.
WhatsApp Web is a computer based extension of the WhatsApp account on your phone.
The messages you send and receive are fully synced between your phone and your computer.
The first requirement to being able to use WhatsApp Web is an active WhatsApp account on your smartphone.
If you cannot connect to WhatsApp on your phone to send and receive messages, then you will not be able to use WhatsApp Web. - According to the WhatsApp FAQ
So basically, it just access your phone to sync data.
And this on Quora discuss the details.