A mole is defined as the number of atoms in an exact mass of which isotope?

Isotope of carbon-12.

Explanation:

The MOLE  is a unit of measurement of the amount of a pure substance which contains exactly the same number of chemical units which could be (Atoms, molecules etc.) posses atoms in exactly grams of isotope of carbon-12 (i.e., ).

So, The mole is the Notation  used for an amount  as same the 12 number is used for dozen for making count of Bananas and many more.

The is known as the fundamental constant names Avogadro's number (), It is in the honor of Italian scientist Amedeo Avogadro.

the answer is neither ball has a charge.

because if they were both positively charged, they will repel each other.

if they were positively and negatively charged, they will seek.

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.