# How can I calculate freezing point depression of Water?

##### 1 Answer

May 2, 2014

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

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: