i have a lenovo gt80 and wondered if i could just buy a 8gb ram chip (this one https/www.amazon.de/gp/product/B00ZUI2TJM/ref=ask_ql_qh_dp_hza) and install it.
i know how to replace it and i wondered if the Mainboard Supports this
Well, your laptop will support up to 16Gb of ram. That being said, there are no guarantees that unmatched ram will work together. Laptops tend to be EXTREMELY picky when it comes to ram, moreso than desktop systems. It may even work with each stick individually, but not with both together.