$5 g$ of a nonvolatile compound (molar mass $50\; g \;mol^{-1}$) is added to 100 g of water . If the decrease of pressure is $11.31$ Torr, the vapour pressure of water will be

$\begin{array}{1 1} 480 \;Torr \\640\;Torr \\840\;Torr\\988\;Torr \end{array}$

Answer :$640\;Torr$