Question

1. There are two stable isotopes of iridium: 191-Ir (190.96 amu) and 192-Ir (192.96 amu). If you were to randomly pick an iridium atom from a large collection of iridium atoms, which isotope are you more likely to select. 2. Why?

Answer 1

you are not mentioning the relative abundance of each isotope
Iridium has two naturally occurring, stable isotopes, 191Ir and 193Ir, with natural abundances of 37.3% and 62.7%, respectively
http://en.wikipedia.org/wiki/Iridium

in this case the most abundant is the one that have more chances to be pick it in a random choice