When an electron is removed from its position in the atom, a hole is created. Then how can a empty space created by removing something can have mass? Apparently what do you mean by "mass of a hole"? Any other answers please?

Mass is resistance to acceleration. F=ma. The hole is over here, you apply an electric field, and it takes a while to get to over there. Might as well say the hole has mass, because it acts like it! It's more practically useful than theoretically precisely correct.

I'm guessing that holes have lower mobility than electrons because the energy that was put into moving the first electron cannot be re-used to move the second electron, instead you have to start over using new energy (in other words, you don't get inertia working for you) -- either that, or it's one of those quirky quantum things, related to the fact that there is only one hole at the top of the energy ladder, or maybe the uncertainty principle: more time means more precise energy... I dunno.

I think you're mixing up holes and positrons.

To get a positron, you take an isolated piece of empty space and "remove" an electron, leaving what Dirac called a hole, and we now call a positron (if I remember right, Dirac referred to a "sea of holes"). Or to put it another way, you pull apart nothing and get two equal and opposite somethings. The electron and the positron both have mass because of the energy you put in to accomplish this separation. In the converse operation, you drop an electron into a positron (or mutually drop both of them into each other) and you get the energy back out in the form of photons (gamma rays).

Second issue: You are correct, when an electron is removed from its position in the atom, a hole is created. This hole doesn't have mass, or maybe from a practical point of view it has mass because it requires force to move it around, but when you think you're "moving the hole" what you're really doing is moving an electron in the opposite direction into the hole, so the mass involved is really that electron's mass.

It is true that a hole is created when an electron is removed from its position. But, since the electron is removed and carries a negative charge(e=0.0000000000000000019C), its opposite charge of equal magnitude is a proton(or a hole) and have a definite mass. Lastly, by mass of a hole we mean the mass of a proton.