3.4 Flashcards
Henry’s Law vs Raoult’s Law?
- Raoult’s for non-volatile solutes in pure liquids (ideal solution).
- Henry’s for volatile solutes in a solution.
How to get mol fraction of solute from Xa?
Not so fast padawan.
Xa is the mol fraction of the solVENT and like any good fraction, is a fraction of 1.
Thus, if the mol frac of solvent is 0.98, mol frac of solute is 0.02. Clever, I know
Thus, Xb = 1 - Xa
The vapour pressure of an aqueous solution at 100°C is 745 torr. Calculate the normal boiling point of the solution. (KB = 0.51 K kg mol-1)
Where am I trying to get to?
ΔTB = KB x b (assume i = 1)
add ΔTB to 100 degrees.
What can we infer about Pa or the vapour pressure at normal boiling point?
Boiling occurs when vapour pressure = atmo pressure
thus, Pa = 760 torr, 1 atm, 101.325KPa… etc
The vapour pressure of an aqueous solution at 100°C is 745 torr.
What do we know about the Pa0?
100 degrees is the normal boiling point of the pure solution, and for Raoult’s Law to apply conditions of constant temp must be met between Pa and Pa0 if they’re to have a constant relationship linked by mole fraction. Basically, This means for an aqueous solution at 100, this is normal boiling point of water aka when water vap P = atmo P thus we know at 100 deg the vapour pressure of the pure solvent (water) (Pa0) = 760 torr/ 1atm, etc etc. Exact value of temp is less relevant so much as the fact that its got to be the same for all values.
But how does finding the mol fraction of the solute from Raoult’s Law (PA = XAPA0) help me find the molality?
mol fraction of solute = n(solute)/(n(solute) + n(solvent)). Thus assume you have 1L of solution, aka 1000g of water, and then you have n(solvent)… solve for n(solute) from here.
actually wait screw the answers to this handout…
If you have mol fraction of solvent and mole of solvent just rearrange in the original form of Xa = nA/(nA + nB) and it will be faster and involve less working.
if its aqueous and you assumed 1L earlier, divide mol by 1Kg… aka no change.
A 0.124 molal CCl3COOH(aq) solution has a freezing point of –0.423 °C. What is the percentage ionisation of the acid? (KF = 1.86 K kg mol-1)
I did this and i = 1.83. How can the percent ionisation be more than 100%????
i = the absolute dissociation: i = 1 + % dissociation
Therefore % diss = i - 1.
What does Raoult’s Law describe?
The vapour pressure of a solution of non-volatile solute.
The stronger the solution, the lower the mole fraction and the lower the vapour pressure of the solution.
Colligative properties are properties of a solution that vary by how concentrated the solution is, but not what is dissolved. Which of the following is NOT a colligative property?
Vapour pressure.
Boiling point.
Melting point.
Osmotic pressure.
Density.
The density of a solution will most likely be different than the density of the solvent, but it will change by different amounts and in different directions depending on what the solute is. Density is not a colligative property.
What is a quantitative heating curve diagram looking forward?
Relationship between T (y axis) and q (X axis). Maps temp changes and phase changes (which occur where there’s flat lines).
What might trip me up?
i: for NaCl fully dissociated, i=2
R: only use 0.0821 for ATM, otherwise 8.314
Which form of Henry’s law to apply when?
i) If a pressurized gas is present above a liquid, some of it will dissolve in the liquid and the solubility S is proportional to the pressure (I assume this is what you mean when you say the solubility formula?)
ii) If there is a dissolved gas in a liquid to make a solution, the solute (i.e. the gas) contributes to the vapour pressure above the solution.
This is case ii, which we cover in the colligative properties lectures: P = k*x where x is the mole fraction and P the vapour pressure due to the gas. As x is unitless, the Henry constant k has units of pressure. In the question you’ve attached, the Henry constant has units of pressure, and you’ve got the mole fraction – so you just need to multiply them.
Why does it matter if its a non-electrolyte? What is an electrolyte?
A simple example of an electrolyte solution is sodium chloride in water. In the presence of water, solid sodium chloride dissociates as it is dissolved, forming an electrolyte solution:
NaCl→Na+ + Cl−
Nonelectrolyte solutions are those in which the solute does not dissociate into ions when dissolved; sugar does not dissociate, for example. The number of moles of dissolved particles is greater for electrolyte solutions, so there will be a greater impact on colligative properties. Thus van hoff no. is prolly 1
The vapor pressure of an electrolytic solution is dependent on the ratio of solute to solvent molecules in a solution.
