The gases hydrogen, oxygen, carbon dioxide, sulphur dioxide and chlorine are arranged in order of their increasing relative molecular masses. Given 8 g of each gas at STP, which gas will contain the least number of molecules and which gas the most?

Moles = $\frac{givenmass}{molecularmass}$
Moles of hydrogen = $\frac{8}{2}$ | molar mass of hydrogen = 2 g
Moles of oxygen = $\frac{8}{32}$ | molar mass of oxygen = 32 g
Moles of carbon dioxide = $\frac{8}{44}$ | molar mass of carbon dioxide = 44 g
Moles of sulphur dioxide = $\frac{8}{96}$ | molar mas of sulphur dioxide = 96 g
Moles of chlorine = $\frac{8}{35.5}$ | molar mass of chlorine = 35.5 g
Molecules are directly proportion to the number of moles of the compound.
As the given mass is same for every gas, moles are inversely proportional to the molar mass.
Thus,
Sulphur dioxide contain least number of molecule because it has highest molar mass and hydrogen contain maximum number of molecule because it has least molar mass.