Question

The experimentally determined molar mass of a non-volatile solute, $Ba{Cl}_2$ in water by Cottrell's method, is:

• A. equal to the calculated molar mass
• B. more than the calculated molar mass
• C. less than the calculated molar mass
• D. double of the calculated molar mass

Answer 1

The correct option is C less than the calculated molar mass

Van't Hoff factor , $i=\frac{\text{no. of moles after dissociation}}{\text{number of moles before dissociation}}$

$i=\frac{\text{Normal molar mass}}{\text{Abnormal molar mass}}$

Now , as following dissociation takes place.

$BaC{l}_2\to B{a}^{2+}+2C{l}^{-}$

It means no. of moles of ions $=3$

So, $i=\frac{3}{1}=\frac{\text{Normal molar mass}}{\text{Abnormal molar mass}}$

As experimentally determined molar mass $=$ abnormal molar mass
So it means :

$3\times$ Experimental molar mass $=$ Normal molar mass
or ,

Experimental molar mass $=\frac{\text{Normal molar mass}}{3}$

It means experimental molar mass is less than the normal or calculated molar mass.