Which of the following substances would you expect to lower the freezing point of water the most if one mole of the substance was present in 1,000 grams of water? (A) MgSO4, (B) NaCl, (C) BaCl2, (D) C6H12O6, (E) KCl.
The freezing point is a colligative property, which is a property of a solution that depends on the number of solute particles but not on the identity of the solute. When a liquid freezes, the particles in the liquid come together to form the solid. If we add a solute to the liquid, the solute particles will interfere with the ability of the liquid particles to come together to form the solid, which will lower the freezing point. The more solute particles there are in the solution, the lower the freezing point will be. Again, it’s the number of solute particles in the solution that determine how low the freezing point will be. It does not matter what the solute particles are.
With this in mind, let’s look through the answer choices to see which one we would expect to lower the freezing point of water the most. The question tells us that one mole of each substance is added to water. So about 6.02 times 10 to the 23 units of each compound are present. However, some of these species will dissociate in water to form even more particles. Magnesium sulfate will dissociate to form magnesium two plus ions and sulfate two minus ions. So, each unit of magnesium sulfate dissociates to form two particles. Each unit of sodium chloride also forms two particles when it dissociates.
Barium chloride dissociates to form Ba2+ ions and two Cl− ions. So, a total of three particles are formed when one unit of barium chloride dissociates. C6H12O6, or glucose, does not dissociate in water, so one unit of glucose just gives one particle in solution. Finally, potassium chloride dissociates to form potassium and chloride ions, forming two particles per unit of potassium chloride. If we start with one mole of each of these substances, barium chloride will dissociate to form the greatest number of particles. As we said earlier, the more solute particles there are, the lower the freezing point will be.
So, of the substances in this question, the one we would expect to lower the freezing point of water the most if one mole of the substance was present in 1,000 grams of water is answer choice (C), BaCl2.