Question

If your cold pack used 100 mL of water, determine the number of grams of the solid salt that you would need to add to the water to lower the temperature from 25.0°C to 3.0°C.

Answer 1

ΔT_F = K_F · b · i,

where:

• ΔT_F, the freezing-point depression, is defined as T_{F (pure solvent)} − T_{F (solution)}.
• K_F, the cryoscopic constant, which is dependent on the properties of the solvent, not the solute. (Note: When conducting experiments, a higher K_F value makes it easier to observe larger drops in the freezing point. For water, K_F = 1.853 K·kg/mol.^[6])
• b is the molality (moles solute per kilogram of solvent)
• i is the van 't Hoff factor (number of ion particles per individual molecule of solute, e.g. i = 2 for NaCl, 3 for BaCl₂)
• Here salt is NaCl.
• i = 2.
• 25-3 = 1.853*2*x/0.1 (molality = no. of moles of solute in 1 kg of solvent and here solvent is water)
• 22 = 18.53*x*2
• x = 0.59
• Mass = 0.59*58.5
• = 17.36