Moles of Cl2 in 20 mL of it at STP =2022400=8.9×10−4 mol
Thus, no. of Cl atoms =moles×NA×atomicity ..... eq. (i)
Atomicity is the number of atoms per molecule of a species.
∴ Atomicity of Cl2=2
Putting values in eq(i), we get;
no. of Cl atoms =8.9×10−4×6.022×1023×2≈1×1021
Thus value of x=1.