Final answer:
To accurately determine the freezing point of a solution, one subtracts the calculated change in freezing point due to the solute from the pure solvent's freezing point. This process requires knowing the molal concentration and considering the colligative properties of the solution.
Step-by-step explanation:
Instead of assuming that the freezing point of the solvent tap water is 0 degree Celsius, we should determine the actual freezing temperature of the solution. This involves taking into account the effect of solutes on the freezing point depression. To find the new freezing point, we subtract the freezing point change from the pure solvent's freezing point. For instance, if the pure freezing temperature is 5.5°C and the observed change is -1.7°C, the new freezing temperature is calculated as 3.8°C.
The colligative property of freezing point depression explains that the presence of a solute lowers the freezing point of the solvent. For example, adding calcium chloride (CaCl₂) to water can significantly lower the freezing temperature, which is useful for de-icing roads. We must consider the impact of the molal concentration, which is directly proportional to the change in the freezing point. If the calculated change in freezing point is 2.1°C, the new freezing point will be -2.1°C assuming the initial freezing point of pure water was 0°C.