The ideal gas law

Tutorial 3

                

A vacuum is established in a laboratory apparatus. The residual pressure measured at $ 25\;^oC $ is $ 10^{- 6} \; mmHg $. Find the number of molecules per $ cm^3 $! Solution (Click): Conversion to Kelvin: ..........$T=$ $298.15\; K$ Conversion to atm: ..........$P=$ $\frac{10^{-6}}{760}\;atm$ Ideal gas law: ..........$P V = nRT$ Calculating the number of moles for $1cm^3=10^{-3}\;L$: ..........$n=$ = $\frac{PV}{RT}$ = $\frac{10^{-6}10^{-3}}{760\cdot0.082\cdot298.15 }$ =$1.70\cdot 10^{-15} \;mol$ Number of molecules:..........Number of molecules = $n\cdot 6.023\cdot 10^{23}$ = $1.70\cdot 10^{-15}\cdot 6.023\cdot 10^{23}$ =$1.02\cdot 10^9 $