Principles of Chemistry I

Effective vs Stated Molalites

The stated molality is the LABEL molality. It is truely the number of moles of solute that were dissolved in 1 kilogram of solvent. It is what you would write on a label for the bottle of solution that was mixed. Very easy to calculate too: Take the mass of the solute used, divide it by its molar mass, and then divide that by the number of kilograms of solvent used.

The effective molality is the molality of the solution that has been ADJUSTED (increased) to reflect the actual number of moles of dissolved species in solution. The effective molality DOES take into account the percent ionization of the solute. For 1:1 electrolytes the effective molality will ALWAYS be equal to whatever the stated molality is PLUS the amount ionized.

EXAMPLE: Given a 0.16 m solution of the weak electrolyte HX. Also given (or determined experimentally) is the fact that HX ionizes by 25% when mixed at 0.16 m. So what is the effective molality?

m_eff = m_stated + (%ion/100)(m_stated)

m_eff = 0.16 + (25/100)(0.16)

m_eff = 0.16 + (0.04)

m_eff = 0.20 m

The effective molality is 25% higher than the stated. WARNING: This is the case for 1:1 electrolytes. 1:2 electrolytes will have MORE added on - double the %ionization to be exact. 1:3 electrolytes will have TRIPLE the %ionization amount added on to the stated molality. For any ratio of an electrolyte, n:m the formula will be:

m_eff = m_stated + (n+m-1)(%ion/100)(m_stated)

Once you DO have the correct m_eff you can put it into the various colligative property equations and get a more accurate measure of the colligative property.

This is important!

Finally, you can REVERSE this whole process and MEASURE the colligative property, and then backtrack to find the percent ionization. Back sure you can work problems BOTH ways.

A few of your homework problems (and most likely an exam problem) require you to consider effective molality instead of the stated (or calculated) molality.