The most commonly used concentration term in chemistry = moles of solute dissolved in 1 L of solution.
Making molar solutions. (see Lecture 13, 20 February)
Example: Make up a 1.00000 L solution of 0.25 M NaCl (note that water is the "default" solvent).
Example: What is the concentration of a solution made by dissolving 10.00 g of KI in enough water to make
Dilution problems (see Lecture 13, 20 February).
Example: What is the mole fraction of a solution of 10.0 moles of glycerol dissolved in 15.0 moles of water?
(10 mol) / (10 mol + 15 mol) = 10/25 = 0.400
Colligative properties (properties which depend only on the number or concentration, not on the type, of particles). [Exchange across surfaces model]
Colligative properties are only strictly followed for ideal solutions. That is, other forces are at work in real solutions, so will get deviations. As a result colligative properties are followed most closely for dilute solutions (e.g. <0.1 M) where solute-solute interactions are minimal.
Raoult's Law: P = XP°, where P = vapor pressure of substance in solution, P° = the vapor pressure of the pure substance and X = its mole fraction. Recall that mole fraction is the number of moles of substance divided by the total number of moles of all substances in the solution (moles solute/(moles solute + moles solvent)) In other words the vapor pressure of a substance in solution is proportional to the molecular fraction or molecular percentage of that substance in the solution.
Example: What is the vapor pressure of water in 80 proof alcohol at 25° C (vapor pressure = 23.76 mmHg).
P = XP°
XH2O = [60g/18.01 g/mol] / [60g/18.01 g/mol + 40g/(2x12.01 + 6x1.008 + 16.00)g/mol = 0.79
P = 0.79 (23.76 mmHg) = 18.77 mmHg = 19 mmHg
Tb = kbm, where m = molality = moles solute/kg solvent, and kb is a constant specific to the solvent.
Which of the following solutions will have the highest boiling point: 3 m glucose or 1 m aluminum chloride?
First need to look at concentration of particles.
glucose is covalent, so 1 m particles,
aluminum chloride is ionic with 1 mole aluminum ions and 3 moles of chloride ions for each mole of AlCl3 = 4 m particles,
Therefore the 1 m aluminum choride solution will have the higher bp.
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© R A Paselk
Last modified 24 April 2015