Carbon has two naturally occurring isotopes: C-12 (natural abundance is 98.93%) and C-13 (natural abundance is 1.07%). How many C-13 atoms are present, on average, in a 30000-atom sample of carbon?

The number of atoms present, on average, will be the natural abundance of the isotope times the number of atoms in the sample = > number of C-13 atoms = C-13 abundance * number of atoms in the sample = 1.07% * 30,000 = 1.07 * 30,000 / 100 = 321 atoms. Answer: 321 atoms.