Question

How many mercury atoms are present in 36 cubic cm at 20 degree celsius

Answer 1

Calculations for number of atoms can be done at STP:
​At STP,
22.4L of mercury contains one mole of mercury that is 6.022$\times$10²³ atoms.

Therefore, 36cm³ or 0.036L of mercury will contain = $\frac{6.022\times {10}^{23}}{22.4}\times 0.036=9.68\times {10}^{20}$