Final answer:
To find the freezing point of a 7.15 g MgCl2 solution, additional data such as total volume is necessary to calculate its molality. The freezing point is affected by freezing point depression, a collative property that depends on the number of dissolved particles in the solution.
Step-by-step explanation:
The freezing point of a solution of MgCl2 is the temperature at which the solution turns solid. The freezing point of pure water is 0°C, but when a solute like MgCl2 is dissolved in water, it will lower the freezing point of the solution. This is known as freezing point depression, which is a collative property. This means that the freezing point depression depends on the number of particles in the solution rather than the type of particles.
Now, MgCl₂ dissociates into Mg²⁺ and 2 Cl⁻ ions in the solution, which means we would expect the van 't Hoff factor (i), which is the number of particles the compound dissociates into, to be ideally 3 (1 Mg²⁺ + 2 Cl⁻). However, due to ion pairing and other interactions in real solutions, the actual van 't Hoff factor might be less than 3, which is observed in the case of MgCl2 solutions. For instance, if we calculate the van 't Hoff factor for a 0.050 m solution of MgCl2 with a measured freezing point of -0.25°C, we find the factor to be approximately 2.7, which is lower than the ideal value of 3.
To determine the freezing point of a 7.15 g solution of MgCl2, we would need additional information, such as the total volume of the solution, to determine its molality, and then apply the freezing point depression formula. However, this information is not provided, so it is not possible to calculate the freezing point without it. The options given do not provide sufficient information to accurately answer this question.