The correct option is A, Isotopes are atoms of an element, which have the same number of protons but different number of neutrons in the nucleus of the atom. Thus, isotopes have the same atomic number but different mass number. Looking at the options given above, you will discover that Chlorine which is given in option A is correct. Both type of chlorine has the same atomic number, which is 17, but they have different atomic masses, the atomic mass of the first one is 35 while that of the second one is 37.<span />
When you are converting grams to moles, the operation that you will be doing is dividing by the molar mass to obtain the amount of moles of the given sample.
