I don't think you can mount an articulating arm on this wall. An articulating arm must be mounted firmly in studs. Aside from finding the studs, you'll have two other problems:
- Your screws need to be two inches long just to reach the studs through the first wall, the space, the plaster, and the lath or other backing. Then you get 1-1/4 or 1-1/2 inch of penetration into the stud. The problem is that you have so much screw, 2 inches, cantilevered out of the stud and acting to damage the part of the screw that is threaded into the wood.
- You'll have to put spacers between the two walls on each screw to prevent them from crushing the outer wall.
All the above issues can be surmounted but it seems inadvisable to me.
Buy a fixed, flat-against-the-wall TV bracket, and for a 10-lb TV you can use high quality drywall anchors if you know how to install them well. Use 6.
Without too much detail, if you insist on using an articulated mount:
- FIND STUDS. Use a 2-inch core bit to make an access port all the way into the stud bay, then a wire hanger to find studs both sides. Do this behind where the bracket will go. Then use a plastic grommet or the drilled out outer layer to patch the hole.
- Use 3-1/2 inch lag screws, at least four of them, in at least two studs. Do not buy a single-stud mounting bracket. They exist. Don't.
- Drill oversized holes for the screws through all layers of wall, all the way to the stud. DOn't penetrate the stud. Then drill appropriate pilot holes (7/32") into the stud for the screw and use plastic spacers to fill the space between the face of the wall and the stud so the bracket won't crush the wall.