Answer
42.8 mol
Work Step by Step
$V= 1.05\times10^{3}\,L$
$P=745\, mmHg\times\frac{1\,atm}{760\, mmHg}=0.980\,atm$
$T= (20+273)K= 293\,K$
$R=0.0821\,L\,atm\,mol^{-1}K^{-1}$
$PV=nRT$ (ideal gas law)
$\implies n=\frac{PV}{RT}=\frac{0.980\,atm\times 1.05\times10^{3}\,L}{(0.0821\,L\,atm\,mol^{-1}K^{-1})(293\,K)}$
$=42.8\,mol$
