Question

How can I calculate molality of ions in solution?

Answer 1

Molality is the moles of ions in solution divided by the kilograms of solvent.

For example, if you dissolve 1.0 moles of NaCl in 1.0 kilogram of solution, you will have 1.0 molal concentration of sodium chloride.

Because sodium chloride not only dissolves in water, but dissociates into ions, each ion, the sodium and the chloride ion will be 1.0 molal.

Consider another ionic compound, calcium chloride, which has the formula, `CaCl_2`. If you 1.0 molal of this compound is dissolved in water, you obtain a 1.0 molal concentration of calcium and 2.0 molal concentration of chloride ions since when calcium chloride dissolves, it dissociates into three ions, one calcium and two chloride ions.

The number of ions impacts colligative properties which in turn affects boiling point elevation, freezing point depression, and vapor pressure.