Naturally occuring chlorine gas has 3 isotopes

35cl-35cl, 35cl-37cl , 37cl-37cl

a) mass of one molecule in terms of amu.
b) mass of one molecule in terms of grams.
c) mass of one mole in terms of grams.
d) Naturally occuring Cl has 75% 35cl and 25% 37cl, calculate the mass of one mole of cl(small 2) gas.

--I didn't really understood these as i am in prep chem and i guess my teacher is too stupid to understand that. He gave us this on first pop quiz when our first chapter was about conversions and basic stuff. Please help or get me started on this at least.