Answer:
Because it wouldn't make any sense
Explanation:
First of all, I think it's important to highlight the definition of isotope.
Isotope (Wikipedia): Variants of a particular chemical element <u>which differ in neutron number</u>...
This means that two isotopes are the same element but have different net mass per atom, due to the different number of neutrons.
Therefore, it's important to make the definition on the same isotope so that the proportion is equal. If the definition would be made on different isotopes, the proportion wouldn't have any sense. Let me be clear with this example:
mass in grams of a C-12 atom = 1.9944235 × 10 ^ -23 g --> this is the mass of a single C atom.
By definition --> 1 mol of anything = 6.02 x 10 ^ 23 particles of anything
Therefore, we know how much a single C atom weights. How many grams do you think that 6.02 x 10 ^ 23 atoms of C (i.e a mol of C) could weight??
1 single C atom ----------------------------- 1.9944235 × 10 ^ -23 g
6.02 x 10 ^ 23 atoms of C ------------ <u>12.006 g !!</u>
These 12 g is the same quantity than above! Therefore, 1 mol of C weights 12 g. If the definition were made with 13 g of C-13 (the other C isotope), these numbers will not be the same --> There would be a contradiction.
Regarding the second question, we need to search Ne-20 atomic mass in grams --> 3,3509177 × 10 ^ -23 g
Hence, if we follow the same rule, the amu would be 1/12 of Ne-20.
[ 3,3509177 × 10 ^ -23 g ] / 12 = 2.79 ^ -24 g
or 0.18 moles whichever your teacher prefers