calculate the number of molecules in 1ml of O₂ at STP

At STP 1 mole of any gas occupies 22.4 L of volume. 
For O₂, the molar mass = 32 g/ mol
This means, 32 g or 1 mole of oxygen gas occupies a volume of = 22.4 L 
1 L = 1000 mL 
This implies, 
22400 mL of volume is occupied by = 1 mol
​1 mL of  volume is occupied by = 1/ 22400 mol
= 4.46 $\times$10^-5 mol
​As, 1 mol = 6.022 $\times$10²³
Therefore, 1 mL of volume is occupied by = 4.46 $\times$10^-5$\times$ 6.022 $\times$10²³ molecules. 
= 26.85 ​ $\times$10¹⁸ molecules
= 2.685 ​​ $\times$10¹⁹ molecules
Hence, number of molecules present in 1 mL of oxygen gas is 2.685 ​​ $\times$10¹⁹