Atomic mass unit is defined as one-twelfth of the actual mass of an atom carbon-12 isotope.
Now atomic mass of an element tells us to how many times an atom of the element is heavier than one-twelfth of an atom of C-12.It can be represented as ,
Atomic mass = Mass of an atom of the element/1/12 * mass of an atom of C12
For ex- atomic mass of Mg is 24u,which means that an atom of Mg is 24 times heavier than 1/12 of mass of a C-12 atom.
In case of molecular mass of a substance (element or compound) it tells us the average relative mass of a molecule of the substance as compared with mass of an atom of C-12 taken as 12u or it tells us the number of times a molecule of the substance is heavier than 1/12 of mas of a C-12 atom.For ex- molecular mass of water is 18u which means a molecule of water is 18 times heavier than 1/12th of mass of a C-12 atom.
Sol-
1 atom of He will have mass = 4u
or, 4u = 1 atom of He
therefore,
52u of He will have no. of atoms = 1/4* 52 = 13 atoms
