For the reaction 3 Cu +8 HNO3 ----> 3Cu(NO3)2 + 2 NO + 4H2O, if you
could drop 12 atoms of copper into nitric acid, how many molecules of
NO would be produced?
a. 2
b. 4
c. 8
d. 12

Hi again (and again, and again...)!!!

3 Cu + 8 HNO3 ----> 3 Cu(NO3)2 + 2 NO + 4 H2O  also means (in a very
microscopic sense) that 3 atoms of Cu will react with ONLY 8 molecules
of HNO3 to obtain 3 molecules of Cu(NO3)2, 2 of NO and 4 of H2O.
12 atoms of Cu are are the quadruple of 3 atoms of Cu.
Then if you could drop 12 (= 4x3) atoms of copper into nitric acid,
they will react with 32 (= 4x8) molecules of HNO3 to obtain 12 (= 4x3)
molecules of
Cu(NO3)2, 8 (= 4x2) of NO and 16 (= 4x4) of H2O.

The correct answer is c; 8 molecules of NO will be produced from 12
atoms of Cu dropped into the nitric acid.

Regards.
livioflores-ga