CHEM1101 Lecture Notes - Lecture 13: Mole Fraction, Vapor Pressure, Partial Pressure
19 May 2018
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Calculate concentrations in molarity, molality, mole fraction, %
w/w and %v/v and perform dilutions
Above liquid, vapour pressure = partial pressure which builds up above surface due to
evaporation
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In closed container at equilibrium, rate of gas evaporation = rate of condensation,
pressure is called the equilibrium vapour pressure, partial pressure of vapour increases
as more and more molecules enter gas phase
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Addition of a non-volatile solute (ie. One that can't evaporate) stabilises liquid
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Solvent molecules have a greater tendency to stay in liquid phase
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So…presence of solute molecules decreases equilibrium vapour pressure of solution,
compared to pure solvent
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Vapour pressure
For dilute solutions
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Screen clipping taken: 11/06/2017 7:50 PM
P solution = vapour pressure of solution
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Xsolvent = mole fraction of solvent
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P solvent = vapour pressure of the pure solvent
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Raoult's law, mole fraction
Mole fraction - fraction of the total number of moles in a mixture
L21
13.1
Sunday, 11 June 2017
6:42 PM
13. Liquids Page 1
Screen clipping taken: 11/06/2017 7:51 PM
Screen clipping taken: 11/06/2017 7:57 PM
Describing concentrations as the ratio of the number of mols of solute per mass (in kg)
of solvent
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Units = mol/kg
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As solute molecules decrease equilibrium vapour pressure, a higher temp is needed to
teach atm pressure
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Bp of a solvent -> increased by presence of solute
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Molality
Screen clipping taken: 11/06/2017 7:58 PM
No. of mols of solute per volume (in L) of solution
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C = n/V
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Mol/L
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Molarity
Weight of each chem used
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Total mass of whole solution = 100g
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With 10% w/w solution of fat
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.e.g. if dissolved 10g of fat in 90g ethanol
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%w/w
13. Liquids Page 2
Where both chemicals are liquid
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e.g. dilute 50ml of acetic acid by adding 50ml of water
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Now 50ml of acetic acid in total volume 100mL
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So acetic acid conc is 50% v/v
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%v/v
Dilution
C1 x V1 = C2 x V2
C1 x V1 + C2 x V2 = C3 x V3
13. Liquids Page 3
Document Summary
Calculate concentrations in molarity, molality, mole fraction, % w/w and %v/v and perform dilutions. Above liquid, vapour pressure = partial pressure which builds up above surface due to evaporation. In closed container at equilibrium, rate of gas evaporation = rate of condensation, pressure is called the equilibrium vapour pressure, partial pressure of vapour increases as more and more molecules enter gas phase. Addition of a non-volatile solute (ie. one that can"t evaporate) stabilises liquid. Solvent molecules have a greater tendency to stay in liquid phase. So p(cid:396)ese(cid:374)(cid:272)e of solute (cid:373)ole(cid:272)ules de(cid:272)(cid:396)eases e(cid:395)uili(cid:271)(cid:396)iu(cid:373) vapou(cid:396) p(cid:396)essu(cid:396)e of solutio(cid:374), compared to pure solvent. P solvent = vapour pressure of the pure solvent. Mole fraction - fraction of the total number of moles in a mixture: liquids page 1. Describing concentrations as the ratio of the number of mols of solute per mass (in kg) of solvent. As solute molecules decrease equilibrium vapour pressure, a higher temp is needed to teach atm pressure.
