Chapter 13 Properties of Gases - Problems - Page 462: 17

$2.49\times10^{19}\,molecules$

$PV=nRT$ (Ideal gas law) $\implies n=\frac{PV}{RT}=\frac{(2.15\times10^{4}\,Pa)(5.00\times10^{-3}\times10^{-3}\,m^{3})}{(8.314472\,J\cdot mol^{-1}K^{-1})(40.0+273.15)K}$ $=4.128776\times10^{-5}\,mol$ Number of molecules= $n\times N_{A}$ $=4.128776\times10^{-5}\times6.022\times10^{23}$ $=2.49\times10^{19}\,molecules$