What is the mass of a magnesium block that measures 2.5cm?

27.0 grams Assuming the block is a cube, it's volume will be 2.5^3 cm^3 which is 2.5 cm * 2.5 cm * 2.5 cm = 15.625 cm^3 Looking up the density of magnesium, its density is 1.738 g/cm^3, so multiply the volume by the density giving 15.625 cm^3 * 1.728 g/cm^3 = 27.000 g