States of Matter (Gaseous and Liquid States)

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