What is the mass, in grams, of a sample of 6.98 * 10^24 atoms of magnesium (Mg) ?

The number of moles in a substance indicates the amount of the substance that contains the same number of particles as 12 g of the Carbon-12 isotope [or equivalent to 6.02 * 10²³ (which is used as a standard in the world of moles).

Now,

if 6.02 * 10²³ atoms are found in 1 mole of Mg

then let 6.98 * 10²⁴ atoms are found in x

⇒ x = (6.98 * 10²⁴) : (6.02 * 10²³)

= 11.595 mol

Also,

Mass = Mole * Molar Mass

∴ Mass of Mg = 11.595 mol * 24 g / mol

= 278.27 g

The mass of a sample of 6.98 * 10 ²⁴ atoms of magnesium (Mg) would contains 278.27 g