#5 is impossible. Martin Gardner has a nice proof of this: observe that each domino must cover a black and a white square, no matter how you orient it. When you remove two opposite squares, you remove two squares of the same colour. Imagine you have put 30 of the dominoes on the board, then two squares remain uncovered. These squares must be the same colour (by the above rules), so there is no way the last domino can cover them both.