What is the smallest mass (kg) required to form a black hole?
To your first question, I’m not sure what the answer would be. From the stand point of General Relativity (the modern theory of gravity which describes/predicts black holes), there is no minimum mass needed to form a black hole. It’s just that the smaller the mass, the smaller the size (e.g. the Schwarzschild radius) of the black hole (in the sense that cutting the mass of a black hole in half would cut it’s radius in half). On the other hand, it’s not clear how this would work with Quantum Theory which is the current theory of how the universe works on small scales. It is an open question of how you reconcile these two theories, and how they interact could conceivably affect the answer. Of course, this never comes up practically because, for instance, the Schwarzschild radius of an electron is of the order 10^-50 meters (compared to the 10^-15 meter size of the nucleus of an atom, is very, very small indeed). In practice, there may be so called “mirco” black holes scattered throughout
