How many moles of oxygen are present in  $400c{m}^3$ sample of the gas at a pressure of 760 mm Hg and a temperature of ${27}^{0}C$ . (The volume of R is given to be  $8.31kPad{m}^3{K}^{-1}mo{l}^{-1}.)$ 

Since the value of R is given in SI units, it is necessary to express the pressure and volume in kPa and  $d{m}^3,$ respectively.
Thus, 
$\begin{array}{l}P=760mm=101.3kPa\\ V=400c{m}^3=400\times {10}^{-3}d{m}^3=0.4d{m}^3\\ T={27}^{0}C=27+273=300K\end{array}$
According to the ideal gas equation 
 $\begin{array}{l}PV=nRT,\\ n=\frac{PV}{RT}=\frac{101.3\times 0.40}{8.31\times 300}=0.0162=1.62\times {10}^{-2}mol\end{array}$