Use the atomic mass of potassium, k, 39.1 g/mol, and the formula n = mass / atomic-mass.
Where n is the number of moles.
n = 250.0 g / 39.1 g/mol = 6.3939 mol.
Now multiply by Avogadro number to find the number of atoms:
6.3939 mol * 6.02*1023 atoms/mol = 38.49 * 10^23 atoms = 3.849 * 10^24.
Answer: 3.85*10^24
