2.1 ã— 1024 atoms of arsenic is how many moles?

1024 / (6.02 × 10^23) = 170 × 10^-23 mol

Ithink this is what you meant. the temperature at which a liquid begins to boil is called the boiling point. the boiling point of water is 212°f or 100°c.  and at that point will start to turn into steam (gas)