What is the mass of 3.01 x 10^23 molecules of oxygen

That number of atoms is called a mole. The number of atoms in a mole is 6.02x 10^23, Avogadro's number. So a mole of oxygen atoms (atomic weight=16 amu) weighs 16 grams. So figure out what proportion your number of oxygen atoms is to 6.02 x 10^23, multiply by 16 grams, and there is your answer.

Ithink it's carbonic acid

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.