Okay, here is what I've been told by my chemistry contact: "I think that symbol is a typo. It should be a plus sign (+) and the equals sign should technically be a right pointing arrow, if the chemical equation was expressed correctly."
So I'm not sure what to tell you. Was this a scanning error? A font error? What is your source? Can you contact the person who wrote the problem to find out what the original looks like or what the symbol means? It surely is not an empty set symbol.
Nemeth has a method to transcribe this which you could use if all else fails. (1246, 14, 4, 34, 12456) Unspaced. Translation: "circle, superposition indicator, slash, termination indicator." That would be strictly following print. (See NC Section 15.9 "Modification by Superposition.")
Because this is not the usual use of this symbol, I suggest explaining in a transcriber's note that it represents a circle with a superposed diagonal line.
Lindy