Mass=density·volume. The density is 2.70g/mL and the volume is 353mL. So you would multiply 2.70g/mL by 353mL which will give you 953.1g. Hope that helps :)
I believe this nuclide is potassium with the symbol K. Atomic number for an element remains constant no matter how the mass changes. From the periodic table, potassium has an atomic number of 19.
