The mole is defined as the amount of a substance containing the same number of particles as exactly 12 g of c-12. the amu is defined as 1/12 of the mass of an atom of c-12. why is it important that both of these definitions reference the same isotope? what would be the result, for example, of defining the mole with respect to c-12, but the amu with respect to ne-20?

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 which differ in neutron number...

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   12.006 g  !!

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

you need to give more information

plasma is the state of matter where the substance is ionized and has no definite shape or volume. the fact of being ionized makes the substance a good electrical conductor.