Question

John dissolves .5g of a white powder in 25g of benzene (FP 5oC) (kf benzene is 5.1) and finds the solution freezes at 3.7oC. Determine the MW of the compound!

Answer 1

$∆T_f=iK_fb$

$K_f=5.1$

$∆T_f=1.3°C$

$b=0.5×1000/25x$

$i=1(assumed)$

$∆T_f=iK_fb$

$1.3=(1×5.1×20)/x$

$x=102/1.3$

$x=molecular weight=78.462g$