which of the following statements is NOT true about one mole of each
of these substances at STP;hydrogen, oxygen, nitrogen?
a.All have the same density  b.All occupy a volume of 22.4L c.All are
diatomic elements  d.All contain Avogadro's number of representative
particles.

Hi again!!

The statement a) is NOT true. 
We know that the 3 gases at STP have the same volume, but because the
concept of molar mass (1 mole of a pure substance has a mass in grams
equal to its molecular mass (MM)) we have that the mole of the 3
different gases have different mass. Such density is mass divided by
the volume we have three different densities.

For more info see this page "The Mole & Molar Mass":
http://dbhs.wvusd.k12.ca.us/Mole/MolarMass.html


Hope this helps.

Regards.
livioflores-ga