CE Chem MC 07

The answer is A.

Denote V cm3 as the molar volume of a gas at room temperature and pressure.
(You make take 24000 cm3 instead to calculate the exact numbers.)

Denote L as the Avogadro number.
(You make take 6.02 x 1023 instead to calculate the exact numbers.)

Consider: No. of moles of a gas = (Volume of the gas) / (Molar volume)

No. of moles of molecules in 100 cm3 of CO2 = 100/V
1 CO2 molecule contains 3 atoms.
No. of moles of atoms in 100 cm3 of CO2 = (100/V) x 3 = 300/V
No. of atoms in 100 cm3 of CO2 = (300V) x L = 300L/V

No. of moles of molecules in 100 cm3 of SO3 = 100/V
1 SO3 molecule contains 4 atoms.
No. of moles of atoms in 100 cm3 of SO3 = (100/V) x 4 = 400/V
No. of atoms in 100 cm3 of SO3 = (400V) x L = 400L/V

No. of moles of molecules in 200 cm3 of O2 = 200/V
1 O2 molecule contains 2 atoms.
No. of moles of atoms in 200 cm3 of O2 = (200/V) x 2 = 400/V
No. of atoms in 200 cm3 of O2 = (400V) x L = 400L/V

Ratio of the number of atoms
(100 cm3 of CO2) : (100 cm3 of SO3) : (200 cm3 of O2)
= (300L/V) : (400L/V) : (400L/V)
= 3 : 4 : 4