Use the ideal gas law, PV=nRTPV=nRT, where nn is moles and RR is the gas constant.
Convert molecules to moles by dividing by (6.022xx10^23"molecules")/(1"mol")6.022×1023molecules1mol.
2.01xx10^23"molecules"xx(1"mol")/(6.022xx10^23"molecules")="0.33378 mol"2.01×1023molecules×1mol6.022×1023molecules=0.33378 mol
(I am keeping a couple of guard digits to reduce rounding errors.)
"STP"STP is "273.15 K"273.15 K and "100 kPa"100 kPa.
Given/Known
P="100 kPa"P=100 kPa
n="0.33378 mol"n=0.33378 mol
R="8.3144598 L kPa K"^(-1) "mol"^(-1)"R=8.3144598 L kPa K−1mol−1
https://en.m.wikipedia.org/wiki/Gas_constant
T="273.15 K"T=273.15 K
Unknown
VV
Equation
PV=nRTPV=nRT
Solution
Rearrange the equation so that VV is isolated and solve.
V=(nRT)/PV=nRTP
V=(0.33378cancel"mol"xx8.3144598 "L" cancel"kPa" cancel("K"^(-1)) cancel("mol"^(-1))xx273.15cancel"K")/(100 cancel"kPa")="7.58 L"