# Can you use Avogadro's hypothesis to explain why Hydrogen is less dense than Helium?

Jun 14, 2014

Let us take one mole of Hydrogen and one mole of Helium at standard temperature and pressure (STP).

According to Avogadro's hypothesis, one mole of gas at STP is 22.4 L. Now the density is a ratio of the mass of the gas in grams compared to the volume of the gas in liters.

$D e n s i t y = \frac{m a s s}{v o l u m e}$ $D = \frac{g}{L}$

Now the mass of one mole of an element is equal to the average atomic mass (amu) of the element from the periodic table. the mass of Hydrogen is 1.001 g/mole and the mass of Helium is 4.002 g/mole.

Therefore the density of each gas can be found by dividing the mass of one mole of the element by the volume of one mole of the gas at STP.

${D}_{H} = \frac{1.001 g}{22.4 L} = 0.0447 \frac{g}{L}$

${D}_{H e} = \frac{4.002 g}{22.4 L} = 0.179 \frac{g}{L}$

Helium is more than 4 times more dense as compared to Hydrogen.