I have laptop Acer Aspire One 722. Specification says that it has 2 DIMM slots, but when I opened it, I found it has only one, and there is a place for second one. I'm just interested, could I add second one DIMM slot? Can I just solder a new one into that slot?
Short answer: No (most likely)
Long answer: The connection from the northbridge chip to the ram slots consists of various supporting hardware (capacitors, resistors ect..). Most likely those components have been omitted as well.
I built computers for a company that used desktop motherboards which only had 2 of the 4 ram slots populated. It was very easy to see that resistors and capacitors around the empty ram slots were also missing.
If you try it, and have success, post your results.
Checkout these pictures from someone who asked the same question.