If you have this Acer Aspire 7 laptop like me, it comes with 8GB of RAM and while it can be expanded to 32GB (some sources say 64GB), the logical expansion is to move from a single 8GB stick to 2x 8GB sticks giving you 16GB. This is the cheapest, and most effective level of upgrade. What you will want is the matching part to what is already there so that you have a matched pair.
The included RAM stick is HMA81GS6JJR8N-VK from Hynix. Just order another one! Should be around $35.