10 mL of hydrogen contains 2×103 molecules of hydrogen at certain pressure and temperature. Calculate the number of molecules of oxygen whose volume is 200 mL at the same temperature and pressure.
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Solution
By ideal gas equation, PV=nRT
V∝n at constant P and T.
But no. of moles ∝ no. of molecules.
Therefore, Volume ∝ No. of molecules.
∴V1V2=M1M2
∴10200=2×103M2
∴M2=4×104 molecules
Therefore, the number of molecules whose volume is 200ml are 4×104 molecules