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Calculate the amount of KCl which must b...

Calculate the amount of KCl which must be added to 1 kg of water so that the freezing point is depressed by 3 K. (K for water = `1.86 K "mol"^(-1)`).

Text Solution

Verified by Experts

KCI is a strong electrolyte and is completely dissociated in water as :
`KCI to K^+ +Cl^-`
Now, 1 mole of KCI will give 2 mole ions. Therefore, the value of Van.t Hoff factor (i) will be equal to 2.
`Delta t=i k_f m `
`K_f =1.86 (K kg "mol"^(-1))`
`Delta T = 3 K,i=2`
`m= (Delta T )/(i K _f ) = ( 3 )/( 2 xx 1.86 ) = 0.81` mole
Thus, 0.81 mole or `0.81 xx 74.5 g = 60-34 g` must be added to 1 kg of water.
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