Question

Amazon it equal to exactly 112th of the mass of one atom of carbon 12

Answer 1

First we have to understand what we're defining with 1/12 of the mass of an atom of C-12. This definition is for the relative atomic mass of any atom.

Carbon-12 contains six protons and six neutrons, a nice even number of both. Since the smallest atom possible consists of one proton, the smallest relative mass of an atom compared to C-12 is [1/12 of C-12].

This convention gives a nice whole number for every relative atomic mass, rather than values to the 6th and 7th decimal place that really don't matter in the quantities that we talk about.

It could be done with Helium, for instance (=1/4 of the mass of He-4), but at the time of the conception of this convention I imagine C-12 was easier to get a hold of and measure.