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