Answer
104 g/mol
Work Step by Step
We find:
$P=743\, mmHg\times\frac{1\,atm}{760\, mmHg}$
$=0.97763\,atm$
$V=115\,mL= 115\times10^{-3}\,L$
$T=(66.3+273)K=339.3\,K$
$PV=nRT\implies n=\frac{PV}{RT}$
$=\frac{(0.97763\,atm)(115\times10^{-3}\,L)}{(0.08206\,L\,atm\,K^{-1}mol^{-1})(339.3\,K)}$
$=0.004038\,mol$
Molar mass $M=\frac{\text{mass in grams}}{\text{number of moles}}$
$=\frac{0.418\,g}{0.004038\,mol}$
$=104\,g/mol$
