This has a startling similarity to the question about the Argo (in the story of Jason and the Argonauts); if a ship on a long journey eventually has every single part of it replaced along the way, is it still the same ship?
The cells in your body are not the same set of cells that were in it 7 years ago. Are you a different person now?
I'd put to you that the question is one of categorisation; what you refer to as a chair, you do so because the common conceptual framework you share with those around you (language) has a concept for this object that can be condensed down to a single syllable and word.
Separating or combining objects at the molecular level (while and engaging intellectual exercise) is fundamentally flawed because we don't see molecules. What we see and interact with is objects. In some ways, a chair is a chair because we all agree it is. Is it indivisible from the rest of the room in which it is placed? on some levels the answer is no but I can pick it up, take it out of the room, place it on the top of a rocket and launch it out into space. It's still a chair, but it's no longer in the room, or even on the Earth.
You could replace each of the legs in sequence. You could re-stuff it. You could irradiate it, changing some of the molecules inside it, but it's still a chair. You could smash it, then it's a broken chair.
Language, when you get right down to it, is about classification of perception. That we have better understanding and tools than we had in the past doesn't change what we can see with the naked eye, and what we can build and touch with our hands.
There is a theory in science called the Boltzmann Brain. This says that it's statistically more likely that we're just brains created with the memories of the past linked to a stream of inputs for the duration of our existence than we are real flesh and blood people. Why? Because that would be a simpler construct than an Earth that has allowed us to evolve on it over 4.5 Bn years of existence.
Let's assume that's the case; does the chair still exist? It does in our minds, and that's all we can claim in the real world as well.
The chair may well exist objectively. It may well (through wave theory or the holographic paradigm) be an indivisible part of the complete universe. It may also exist as nothing more than a memory in our minds with ongoing stimulus convincing us of its reality. Regardless of which of these is true, the only awareness we have of the chair is as an object that our senses have told us exists, and as such it's real for us. That's all we're ever likely to be able to prove. It's persistence of reality (the fact that it's still there when we get home from work for instance) may lead us to be able to infer that it's objectively more than an object in our memory, but can you be sure?