How many carbon atoms are there in a diamond (pure carbon) with a mass of 51 mg?

2.557x10^23 atoms
 
The atomic mass of carbon is 12.0107. So the number of moles of carbon in the 51 mg diamond is 0.051 / 12.0107 = 0.004246 moles Now multiply by avogadro's number (6.0221409x10^23) to get the number of atoms. Giving 0.004246 * 6.0221409x10^23 = 2.5571298x10^21 Rounded to a reasonable precision gives you 2.557x10^23 atoms

The correct answer is "atomic weight".