How many atoms are in a sample of chromium with a mass of 13 grams?

Let's find the number of moles first.

Number of moles = mass/molar mass

Molar mass = 51.9961

Number of moles = 13 / 51.9961 = 0.2500.

1 mole of any particle contains 6.02 * 10^ (23) atoms.

Hence 0.2500 moles contains X

Hence X = 0.25 * 6.022 * 10^ (23) = 1.5055 * 10^ (23)