The atomic mass of carbon is 12.0. Using the atomic number, determine the number of neutrons present in one carbon atom.

The atomic mass of carbon = 12

and its atomic number = 6

The atomic number of an element is the number of protons that it has. Thus, carbon has 6 protons. Atomic mass, on the other hand, is generally taken to be the sum total of the number of protons and neutrons that it has. Thus, carbon has a total of 12 neutrons and protons. 

Atomic mass = number of protons + number of neutrons

= atomic number + number of neutrons

Since, we already know that carbon has 6 protons (from its atomic number), we can determine the number of neutrons as,

number of neutrons = atomic mass - atomic number 

= 12 - 6 = 6.

Thus, a carbon atom has 6 neutrons. Note that we are talking about the most abundant isotope of carbon, which has an atomic mass of 12. Carbon has other isotopes: carbon-13 and carbon-14. These isotopes contain different number of neutrons than carbon-12.

Hope this helps.