Chapter 14. Solutions. “how things dissolve”. – homogeneous mixtures of 2 or more substances – does the dissolving (H 2 O is the universal solvent) – what gets dissolved 3 things happen when dissolving occurs: 1. solvent molecules split up 2. solute units 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)
Non-electrolytes undergo __________ __________
Ex. oil/water vs. oil/benzene
~ liquid in liquid – often volumes are not additive, called __________.
Why? intermolecular attractions
50 mL H2O + 50 mL methanol ≈ 97 mL total
gas in liquid – same attraction
n = moles of soluteV = Liters of solution
n = samekg = mass of solvent
xa = naxb = nbetc…
Ex. 1) What is the molality and mole fraction of 50.0 g potassium chloride in 425 g water?
-- 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
__________ – 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 = molalityk = constant
i = van’thoff factor ~ measures the extent of ionization (number of dissolved particles)
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?
π = symbol for osmotic pressure
M = molarity T = temp
R = 0.082057 L atm/mol K
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?