The number of chlorine atoms in 20mL of chlorine gas at STP is 1×1021 Given values are: At STP, pressure of gas, p=1atm Temperature of gas, T=273K Volume of gas, V=20mL=20∕1000L Gas constant, R=0.083L atm ∕Kmol Ideal gas equation: pV=nRT On putting given values of p,V and T in Eq. (i) 1.0×‌
20
1000
=‌
N
6.023×1023
×0.083×273 n= number of molecules, Avogadro's constant, NA=6.023×1023 Number of Cl2 molecules, N=5.3×1020 Number of Cl atoms in 1 molecule of chlorine gas =2 Number of Cl atoms in 5.3×1020 molecules =5.3×1020×2=1×1021