Molality

[BiomajorPreDent]

When calculating molality, which is moles of solute over kg of solvent, do you ever have to worry about how many particles are dissolved in the solution?

When you use molality in freezing pt depression and boil pt elevation problems-thats the only time you need to worry about how many particles are dissolved-with molality correct?

(I always get confused about when to worry about dissociation and multplying that by number of moles etc. like in osmotic pressure..which I know is molarity not molality but just for an example)

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[pbure9]

When calculating molality, which is moles of solute over kg of solvent, do you ever have to worry about how many particles are dissolved in the solution?

When you use molality in freezing pt depression and boil pt elevation problems-thats the only time you need to worry about how many particles are dissolved-with molality correct?

(I always get confused about when to worry about dissociation and multplying that by number of moles etc. like in osmotic pressure..which I know is molarity not molality but just for an example)

you're right. Calculating molality is just moles of solute over kg solvent. No need for dissociations here. But it's essential when using freezing point depression or boiling point elevation. Can't think when else you use it off the top of my head. And yea, osmotic pressure is another time you account for i.

 

[iwannabadentist]

you only need to worry about the number of particles when it comes to colligative properties of solutions. these include vapor pressure, boiling point elevation, freezing point depression and osmotic pressure.

 

[BiomajorPreDent]

ok thanks

and could you explain Raoult's law in laymens terms? lol

 

[pbure9]

ok thanks

and could you explain Raoult's law in laymens terms? lol

Just did a problem using it lol.

It basically allows you to find the vapor pressure of a solution after you add a solute to it.

The formula is VP = Pure Vapor Pressure x Mole Fraction solvent

So you take the mole fraction of the solvent over the total and multiply by the pure vapor pressure of the solution

Raoult's Law is only ideal though for solutes that don't dissociate, so I don't think the DAT would use NaCl. They'd probably use something like glucose. Because as iwannabadentist said, i is important in vapor pressure as well.

EDIT: Wondering if anyone has seen a Raoult's Law problem where the solute dissociates.