Question

The vapor pressure of 1 M of KI solution is ________ 1 M of glucose solution. This is because vapor pressure depends only on the _________.
Which option correctly completes this statement?
a) the same as; concentration of the solute
b) less than; number of particles of solute
c) less than; molar mass of the solute
d) higher than; molar mass of the solute

Answer 1

Answer: b) less than; number of particles of solute

Explanation:

As the relative lowering of vapor pressure is directly proportional to the amount of dissolved solute.

The formula for relative lowering of vapor pressure will be,

$\frac{p^o-p_s}{p^o}=i\times x_2$

where,

$\frac{p^o-p_s}{p^o}$= relative lowering in vapor pressure

i = Van'T Hoff factor

$x_2$ = mole fraction of solute  =$\frac{\text {moles of solute}}{\text {total moles}}$

a) 1 M KI

i= 2 as it is a electrolyte and dissociates to give two ions.

$KI\rightarrow K^++I^-$

b) 1 M glucose

i= 1 as it is a  non electrolyte and does not dissociate to give ions.

Thus as number of particles in 1 M KI is higher, the lowering in vapor pressure will be more.