Explain why the mole is used as a counting unit for mass

It isn't.  

Explanation:

It is a counting unit for "things" — atoms, molecules, ions, etc. —  and these things all have different masses.

For example, a chemist may know that one atom of Cu reacts with one atom of S to form one molecule of CuS.

Cu + S ⟶ CuS

She knows that 1 mol of Cu (6.022 × 10²³ atoms of Cu) has a mass of 63.55 g and 1 mol of S (6.022 × 10²³ atoms of S) has a mass of 32.06 g.

By measuring  out 63.55 g of Cu and 32.06 g of S, she knows that there are just enough atoms of each element to react with each other.

Thus, the mole has enabled her to count atoms.

you need to take a look at the periodic system in order to sort these elements into pairs that would most likely exhibit similar chemical properties. first, you need to find elements that are found in the same group (groups are vertical in the periodic system), and then to see whether they have the same valence number. if they meet these criteria, they will have similar properties.

the first pair: ne (neon) and kr (krypton), are noble gases found in group 8

the second pair: li (lithium) and k (potassium), found in group 1

the third pair: s (sulfur) and te (tellurium), found in group 6

c.

explanation: