Compared to a 0.500 mole sample of Helium gas at 0.00°C and 760. mmHg, a 0.500 mole sample of
Hydrogen at 298 K and 101.3 kPa contains
A) molecules having higher average kinetic energy
B) fewer molecules
C) more molecules
D) molecules having lower average kinetic energy

answer: i would say b.

explanation:

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.