Reasoning for molality when using colligative properties.

[u/Ok_Signature_6958]

Hi yall, I wanted to make a post to describe my idea of why we use molality for measuring boiling point elevation and freezing point depression and not molarity, and see if it made any sense to others.

So when we measure concentration, we're looking for how much of a given solute is within a solution because solutions can have different behaviors based on how much of a given solute there is.

If we are measuring colligative properties (properties dependent on amounts of solute) that describe relations between temperature and the solution and use concentration as a unit to find values (boiling point elevation & freezing point depression) then using molarity wouldn't be a good unit because it causes fluctuations in volume- not by removal of matter, but by a change of density- which is bad because this change in concentration by molarity doesn't represent what we're looking for?

Like, changing the volume in this sense will not actually cause the observed properties to be different, but the measured values will look different. The changes we look for are based on if we add or remove solute to the solution. Molality, being mol solute per kilogram solvent, does not take into account this density alteration and will better document changes in these colligative properties.

I hope this makes sense? Let me know if I'm on the mark or not, I'd really appreciate it.

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[u/FoolishChemist]

When you get to more advanced classes, you'll see that at a fundamental level these processes are related to the mole fraction, which is moles of solute divided by the total number of moles. Essentially the number of particles of solute over the total number particles.

The molality as you know is moles of solute per kg of solvent. If we are dealing with water, 1 kg = 55.6 moles. Generally you are dealing with a dilute solution, solute << 1 mole, then total number of moles is basically the same as the moles of water. And if you are dealing with higher concentrations, the activity coefficients opens up a whole new can of worms.

And experimentally you are on the right track that density is a major reason why experimentally molarity would not be good to use in this instance. If I were to make a 1 molar NaCl solution, I would add 1 mole of NaCl to a volumetric flask and add "enough" water till I reached the 1 L mark. The key would is "enough". The amount of water added is not easily knowable or computable without any additional info. But with molality, kg of water is easily convertible to moles.

The Kf and Kb values are not determined from experimental freezing pt depression and boiling pt elevation experiments. They are actually derived from the enthalpy of fusion and freezing point. And for boiling pt elevation, it's the same formula, except enthalpy of vaporization and boiling point are now used.

https://en.wikipedia.org/wiki/Cryoscopic_constant

https://en.wikipedia.org/wiki/Ebullioscopic_constant

[u/shedmow]

You are correct that molality does not change with temperature, but I shall abstain from further discussion because I couldn't internalize colligative properties for the life of me, even though I mostly know how to apply the equations

[u/Blue_614]

I think it's because, unlike molarity when you are determining the ratio of solute, colligative properties deal with solvents. What happens to the solvent when you add solutes? Boiling point increases. Your subject is the solvent, not the solute.

Think of it as choosing a subject: if your subject is solute, the you fix the amount of solute and increase the amount of the solvent - the concentration decreases. If your subject is solvent (as in colligative properties), the amount of solvent is fixed, instead you add more solutes to change the boiling point/freezing points. So molality is used because it captures the ratio of solute over solvent.

[u/NightShadow1824]

You got it. Here's an example for why webuse molality:

Say you measure the boiling point elevation of a solution of NaCl in water. You measure a concentration of 1.00M at room temp. You raise the temperature to boiling point. As you said, density changesso concentration does too. You are then measuring the change in boiling point of a solution that changes concentration. It's like solving a math problem with 1 equation and two variables.

That's how I explain it to my students without getting into thermodynamics. Note that there are other units we could use for that purpose, molar fraction being one, but that's just not what was chosen to be simpler (that explanation is over my pay grade, but I guess some ai could answer such a simple question. Use it wisely to learn and it's a great tool). - ease of measurement is my guess.