Which one of the following pair has the same number of ions?
(a)1 mole of nacl and 1 mole of mgcl2.
(b)1/2 mole of nacl and 1/2 mole ofmgcl2
(c)1/2 mole of nacl and 1/3 mole of mgcl3
(d)1/3 mole of nacl and 1/2 mole of mgcl2

Hahah none of the above but IF (c) is
 
1/2 mole of NaCl and 1/3 mole of MgCl2 instead,

then C is the right ans :)

Which one of the following pair has the same number of ions?

(c)1/2 mole of NaCl and 1/3 mole of MgCl3

Hope this helps!

Isotopes have the same number of protons, but a different number of neutrons. This means that they have different masses, but retain the same chemical name. For this problem, Pb-206 and Pb-208 are both lead, but have different masses (206 and 208), so they are isotopes.
C-14 and N-14 have a different number of protons (they are not the same element).O2 and O3 have a different number of molecules.S and S(2-) are the same element and have the same number of neutrons, but S(2-) just has 2 more electrons that give it the charge.