How can I calculate freezing point depression of Water?

1 Answer

You will need to use the equation:
ΔTf = Kf mi

ΔTf = how much the freezing point is lowered
Kf = freezing point depression constant of water (-1.86°C/m)
m = the molality of the solution (mol solute/kg solvent)
i = the number of dissolved particles the solute produces

The i for any molecular compound is 1

For ionic compounds, you'll need to know how many ions make up a formula unit.
NaCl breaks into two ions so its i value is 2
#CaCl_2# breaks into three ions so its i value is 3

One you calculate the freezing point depression you've already found the new freezing temperature since water freezes at 0°C.

Here is a video with a couple examples: