How much uranium is left if a solid piece exists for one half-life?

Half life is the time taken for half the atoms to decay. Whatever mass you start with, if it is a sample consisting of one pure uranium isotope, you will have half that mass of uranium after one half life. The piece of metal will not weigh half of the original mass, because the decay products will be there. In practice, a piece of uranium usually consists of a mixture of isotopes with different half lives.