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 × 10²⁴ atoms of magnesium (Mg) would contains   278.27 g