Chapter 14. Solutions. “how things dissolve”. Solutions – homogeneous mixtures of 2 or more substances Solvent – does the dissolving (H 2 O is the universal solvent) Solute – what gets dissolved 3 things happen when dissolving occurs: 1 . solvent molecules split up
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3 things happen when dissolving occurs:
1. solvent molecules split up
2. solute units split up
3. Form solvent/solute attractions
NaCl(s) Na+(aq) + Cl-(aq)
nonpolar – molecules split up b/c of the weak attraction into smallest molecule unit
Non-electrolytes undergo molecular solvation
Ex. oil/water vs. oil/benzene
~ liquid in liquid – often volumes are not additive, called miscible.
Why? intermolecular attractions
50 mL H2O + 50 mL methanol ≈ 97 mL total
gas in liquid – same attraction
Saturated sol’n – a sol’n has its maximum amount of solute dissolved in it. You always see un-dissolved solute. (ex. 40.0g NaCl in room temp. water) When a sol’n is saturated, the solute is still dissolving – it’s just that the solute is also re-crystallizing out of the sol’n at the same rate, so there is no apparent change.
n = moles of solute V = Liters of solution
n = same kg = mass of solvent
xa = naxb = nb etc…
-- Pvap goes down with increase in concentration.
Water vs. water + sugar *more sugar
higher Pvap vs. lower Pvap
In solns, fewer molecules have the chance to turn into vapor
Raoult’s Law – the vapor pressure of an ideal solution is directly proportional to the mole fraction of the solvent in the solution
Pvap = (Pvapo)(xsolvent)
~ ideal gases obey PV = nRT very well
~ Raoult’s Law is close to ideal when the solute and solvent have very similar I.F.
pg. 563 & next slide show a phase diagram comparison between pure solvent and a solution
(s.e. dissociates thus higher B.P.)
(w.e. slightly soluble so higher B.P.)
∆T = change in temp. m = molality k = constant
i = van’thofffactor ~ measures the extent of ionization (number of dissolved particles)
π= symbol for osmotic pressure
M = molarity T = temp
R = 0.082057 L atm/mol K
Ex. 1) What is the molality and mole fraction of 50.0 g potassium chloride in 425 g water?
Ex. 2) 15.0 g of ethanol is dissolved in 750. g of formic acid. The freezing point of the solution is 7.20oC. The freezing point of pure formic acid is 8.40oC. What is kf for formic acid?
Ex. 3) What is the molar mass (grams/mole) of 30.0 mL of an unknown substance if 0.300 g of that substance has an osmotic pressure of 0.400kPa at room temperature?
Medicines that are injected into humans such as shots must be at the same concentration as the existing chemical in our blood. These solutions are called isotonic. For example, if the medicine contains potassium ions, they must be the same concentration as the potassium ions in our blood. Why do medicines have to be made that way?