How many atoms are in a sample of uranium with a mass of 789 g.

Number of atoms : 1.996 x 10²⁴

The mole is the number of particles(molecules, atoms, ions) contained in a substance  

1 mol = 6.02.10²³ particles

Can be formulated

N=n x No

N = number of particles

n = mol

No = Avogadro's = 6.02.10²³

mol of Uranium with mass=789 g (Ar U=238,02891 u) :

Number of atoms :

lithosphere and asthenosphere