How do you find the mass of a sample of 1.72 x 1023 atoms of potassium?

1 mole K atoms = 39.0983g K (atomic weight in grams)

1 mole K atoms = 6.022 x 1023 atoms K (Avogadro's number)

Convert known atoms to moles.

1.72 x 1023 atoms K x (1mol K/6.022 x 1023 atoms K) = 0.286mol K

Convert moles to mass in grams.

0.286mol K x (39.0983g K/1mol K) = 11.2g K