Question

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

Answer 1

Mass of Lithium = 1443.73 g ≈ 1.4 kg

Further explanation  

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) :

$\tt n=\dfrac{N}{No}\\\\n=\dfrac{1.25\times 10^{26}}{6.02\times 10^{23}}\\\\n=2.08\times 10^2$

mass Li (Ar=6.941 g/mol)

$\tt mass=mol\times Ar\\\\mass=208\times 6,941\\\\mass=1443.73~g$