Question

Calculate the amount of NaCl which must be added to the 1kg of water so that the freezing point of water depressed by 3k

Answer 1

Answer:

175.5 grams.

Explanation:

as sodium chloride is a completly dissociating salt, then vant hoff factor i=1.

So, ΔT= K × molality.

There comes the answer from formula directly.

Answer 2

Answer: 47.2 g

Explanation:

$\Delta T_f=i\times k_f\times m$

$\Delta T_f$ = change in freezing point  

i = Van'T Hoff factor  

For $NaCl$

i= 2 as it is a electrolyte and dissociate to give 2 ions.

$NaCl\rightarrow Na^{+}+Cl^{-}$

$k_f$ = freezing point constant  = $1.86^0C/m$

m = molality  

$\Delta T_f=i\times K_f\times \frac{\text{mass of solute}}{\text{molar mass of solute}\times \text{weight of solvent in kg}}$

Weight of solvent (water)= 1 kg

Molar mass of solute (NaCl) = 58.5 g/mol

Mass of  solute (NaCl) = ?

$3=2\times 1.86\times \frac{xg}{58.5g/mol\times 1kg}$

$x=47.2g$

47.2 g of NaCl which must be added to the 1kg of water so that the freezing point of water depressed by 3 K.