How many grams of lithium are in 1.25 x 10^26 atoms of Li?

Mass of Lithium = 1443.73 g ≈ 1.4 kg

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

The number of atoms of Li = N=1.25 x 10²⁶

mol Li(n) :

mass Li (Ar=6.941 g/mol)