....

Molar volume is the volume of one mole of an ideal gas. Under standard pressure and temperature it is 22.4 liters.
"Equal volumes of different gases at the same temperature and pressure contain the same number of particles" (Avogadro´s law).
The number of particles in 24 l at room temperature and usual pressure equals 1 mol = 6x10²³.
That means that 2.4 ml air or 2.4 ml hydrogen in experiments contain1/10.000 mol = 6x10¹⁹ particles.