1.

Which relation is/are correct for an aqueous dilute solution of K_3PO_4 if its degree of dissociation is alpha?

Answer»

`(DeltaP)/P^@ = ("Molality" xx 18 xx (1+3alpha))/1000`
`DeltaT_(F(obs))=K_f xx "molality" xx (1+alpha)`
`(DeltaP)/P^@ = (DeltaT_(f(obs))xx18)/(K_f xx 1000)`
`M_w` of `K_3PO_4 = M_(w_(obs))xx(1+3alpha)`

Solution :`{:(K_3PO_4 , to , 3K^(+) +, PO_4^(3-)),((1-alpha),,3alpha,alpha):}`
Total no. of particles = `1-alpha + 3alpha + alpha = 1+ 3alpha`
`(DeltaP)/P^@=x_2=n_2/n_1=(n_2xxM_(w_1)xx1000)/(W_1xx1000)=("Molality" xx M_(w_1))/1000`
For electrolyte , `(DeltaP)/P^@= ("Molality" xx 18)/1000xx(1+3alpha)` ...(i)
(`M_(w_1)` = 18 , for `H_2O` )
`DeltaT_(f(obs))=K_fxx"molality"xx(1+3alpha)` ....(ii)
Dividing eq. (i) by (ii)
`(DeltaP)/P^@=(DeltaT_(f(obs))xx18)/(K_f xx 1000)`
`i=(1+3alpha)="Normal molecular weight"/"Observed molecular weight"`
Therefore, molecular weight of `K_3PO_4=M_(w(obs))xx(1+3alpha)`


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