If you want to lower water's freezing point 18 degrees C by adding NaCl, what must the molality of the salt solution be? (K(f) for water ris 1.86 degree C/m)

4.84 m

Explanation:

To solve this problem we need Frezing point depression.

Formula is: ΔT = Kf . m . i

ΔT = 18°C

This is data. ΔT is the difference, Freezing T° of pure solute - Freezing T° of solution.

Kf = 1.86 °C/m

m = molality. Our unknown.

i = Ions dissolved in solution, also called Van't Hoff factor.

NaCl → Na⁻  +  Cl⁻

Sodium chloride can produce 2 moles of ions.

We replace data: 18°C = 1.86 °C/m  .  m   .  2

18°C / (1.86 °C/m) . 2 = m

molality = 4.84

the answer is na (sodium)

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