2.78 in is equivalent to 7.06 cm
volume of the cube = (7.06)^3 = 351.9 cm^3
density can be calculated using the following rule:
density = mass / volume
therefore:
mass = density x volume
we have both the density and the volume, therefore, we can substitute to get the mass of the cube as follows:
mass of cube = 4.5 x 351.9 = 1583.5 grams
from the periodic table: mass of titanium = 47.88 grams/mole
to know the number of moles, we simply divide the mass by the molar mass.
number of moles in cube = 1583.5 / 47.88 = 33.1 moles
Each mole contains Avogadro's number of atoms, therefore:
number of atoms in 33.1 moles = 33.1 x 6.02 x 10^23 = 1.99 x 10^25 atoms