question_answer 45)

19.5 g of CH2FCOOH is dissolved in 500 g of water. The depression in the freezing point observed is 1.0°C. Calculate the van't Hoff factor and dissociation constant of fluoroacetic acid. Kf for water is 1.86 K kg mol–1.

Answer:

Step I. Calculation of van't Hoff factor (i) for the acid                                   ...(i) K kg mol-1, M2 = 78 g mol-1, w2 = 19.5 g w1 = 500 g = 0.5 kg   Put these values in equation (i) = 1.0753 Step II. Calculation of van't Hoff factor Suppose degree of dissociation at the given concentration is . CH2FCOOH  CH2FCOO?  + H+  Initial conc.                         1 mot L?1              0              0 No. of moles after dissociation             (1 ? )                            Total no. of moles after dissociation        = 1 + or              Step III. Calculation of dissociation constant for the acid CH2FCOOH  CH­2 FCOO- + H+ Initial colic.          C mol L?1                      0              0 At equilibrium   C(1 ? )         C        C                 But C (molality) = 0.5 m Ka = C2 = (0.5) (0.0753)2 = 2.835 x 10-3