Question

How many grams of NaCl should be added to 500 g of water to produce a solution which freezes at –5.00 C? (The K f for water is 1.86°C/m.) Please provide step by step.

Answer 1

Answer:

39.31 grams of NaCl

Explanation:

Let x be the mass of NaCl added.

Decrease in freezing point = ΔTf = 5°C

Kf for water = 1.86 °C/m

Molality of the solution = m = $\frac{Number\ of\ moles\ of\ solute}{Mass\ of\ solvent\ in\ kg}$

i = Vant hoff factor  = $\frac{Observed\ number\ of\ particles}{Theoretical\ number\ of\ particles}$

Formula for depression in freezing point:

ΔTf = i×Kf×m

i for NaCl = 2 (As NaCl dissociates into Na+ and Cl- ions in solution)

molar mass of NaCl = 58.5 g/mol

$5=2\times1.86\times\frac{x\times1000}{58.5\times500}$

$x=39.31\ grams$