Chemistry and Chemical Reactivity (9th Edition)

$7.72×10^{17}\ molecules/cm^3$
$PV=nRT$ $n/V=P/RT$ $n/V=(23.8/760×1\ atm)/(0.082\times 298)$ $n/V=1.28×10^{-3}\ mol/L$ Using this value, we obtain: $1.28×10^{-3}\ mol/L×1\ L/1000\ cm^3×6.022×10^{23}\ molecule/mol=$ $7.72×10^{17}\ molecules/cm^3$