Q

19.5 g of CH_2 FCOOH is dissolved in 500 g of water.The depression in the freezing point of water observed is 1.0 degree C.Calculate the van't Hoff factor and dissociation constant of fluoroacetic acid.

2.33    $\inline 19.5 \; g$ of $\inline CH_{2}FCOOH$ is dissolved in $\inline 500\; g$  of water. The depression in the freezing point of water observed is $\inline 1.0^{\circ}C$. Calculate the van’t  Hoff factor and dissociation constant of fluoroacetic acid.

Views

Firstly we need to calculate molality in order to get vant's hoff factor.

So moles of CH2FCOOH :

$\frac{19.5}{78} = 0.25$

We need to assume volume of solution to be nearly equal to 500 mL.         (as 500 g water is present)

Now, we know that :            $\Delta T_f = i\ K_f\ m$

or                                           $i = \frac{1}{0.93} = 1.0753$

Now for dissociation constant :-

$a = i - 1 = 1.0753 - 1 = 0.0753$

and,                                            $K_a = \frac{Ca^2}{1-a}$

Put values of C and a in the above equation, we get :

$K_a = 3\times10^{-3}$

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