I'm doing a science project for my school. I experimentally measured the melting point of water-salt solutions, and I want to compare my results to theoretical melting points, but I encountered a problem.
I used this equation to calculate the melting point depression: $\Delta T = b \cdot i \cdot K_\mathrm{f}$. I also compared my data to a phase diagram. I expected my measured melting point would be on the liquid line. But these two methods give me a different prediction. A phase diagram tells me that $\ce{CaCl2}$ would be much more effective at lowering the freezing point of water while the above equation tells me otherwise.
Here is a comparison: Purple indicates the melting point calculated with the equation. The black line is the liquid line on the phase diagram.
Which method is correct and why is there such a big difference?
(I didn't encounter this problem with $\ce{NaCl}$ solutions, the two methods gave more or less the same predictions)
EDIT: my math was wrong on the graph i posted, here is the corrected graph: