How many molecules are there in an ideal gas with a volume of 1.99 L at STP?

Answer 1

PV = nRT
=> n = PV/RT, where n = number of moles

Volume = 1.99L
Pressure = 1 bar
Temperature = 273.15 K
Remember to take the value of R which corresponds to all these units.

Use the above equation to solve for 'n'.

Number of molecules = n * Avogadro's number

Answer 2

You can look at this in a slightly different fashion.
You know 1 mol gas occupies 22.4 L at STP so 1.99 L/22.4 L = number mols.
Then mols x 6.022E23 = number of molecules.