Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2011

Lecture Notes 33: 15 April

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Solutions

Solution Concentrations-a Review & Some New Stuff.

Solutions: a solution occurs when one chemical is completely dissolved or dispersed in another. We most commonly think of solutions as being liquid, but solid solutions also occur, such as the various metal alloys like steel, brass and bronze.

In a solution the substance present in highest concentration is considered to be the solvent, while components in lesser amounts are considered to be solutes. If you dissolve a sugar cube in water you get a sugar solution, where water is the solvent, and sugar is the solute.

FYI Example: What is the solvent in 80 proof rum: 80 proof = 40% alcohol in water, so water is the solvent. What is the solvent in 151 proof rum: 151 proof = 75.5% alcohol in water, so alcohol is the solvent.

 Solubility

All gases are completely soluble in each other.

Liquid solutions

"Like dissolves like."

• Solvation - the process of surrounding a solute with solvent molecules).
  • Hydration (solvation with water).
• Saturation - the maximum amount of solute which can be in solution in equilibria with its pure state.
  • Supersaturation - dissolving solute in excess of saturation - unstable to the addition of solute (carbonated beverages, honey, etc.) Video
• Temperature effects:
  • Gases decrease in solubility with increasing temperature. [example of oxygen solubility]
  • Many solids increase in solubility with increasing temperature, but not always true.
• Pressure effects:
  • Gases increase in solubility with increasing pressure (Henry's Law: P = kC, where k is a constant specific to the solution). [example of carbonated beverages - p 493 in Zumdahl).
    • Note that Henry's Law only holds for gases that are non-reactive with the solvent - e.g. works for oxygen and nitrogen in water, does not work for HCl and water (goes to H⁺ + Cl^-)
  • Solubility of solids generally uneffected by pressure, since both liquids and solids essentially incompressible.

 Concentration Measures

Concentration Terms

Percent Concentration

• Mass percent
  • ppt = parts/thousand (1mg/L of water); ppb = parts/billion (1 microgram/L of water)
• Volume percent

Molarity = moles of solute dissolved in 1 L of solution.

The most commonly used concentration term in chemistry = moles of solute dissolved in 1 L of solution.

Two types of situation arise giving two kinds of problems:

Making molar solutions.

FYI Example: Make up a 1.00000 L solution of 0.25 M NaCl (note that water is the "default" solvent). First weigh out o.25 moles of NaCl = (0.25 mole)(22.99 g + 35.45 g)/mole = 14.61 g Example: What is the concentration of a solution made by dissolving 10.00 g of KI in enough water to make 1.00000 L? First need to find the number of moles of KI: (10.00 g) / ({39.10 g+ 126.9 g}/mole) = 6.135 x 10-2 mole Thus the concentration will be 6.135 x 10-2 M

Dilution problems (see Lecture 12, 14 February).

Molality = moles of solute dissolved in 1 kg of solvent.

Mole fraction = moles of solute dissolved in total moles of solution = n_a /n

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

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.

Be able to solve problems for:

Vapor pressure lowering

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 (X_H2O = 0.79) at 25° C (vapor pressure = 23.76 mmHg).

P = XP°

X = [60g/18.01 g/mol] / [60g/18.01 g/mol + 40g/(2x12.01 + 6x1.008 + 16.00)g/mol

Boiling point elevation

T_b = k_bm, where m = molality = moles solute/kg solvent, and k_b 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 chlroide ions for each mole of AlCl₃ = 4 m particles,

Therefore the 1 m aluminum choride solution will have the higher bp.

Freezing point depression

T_f = -k_fm, where m = molality = moles solute/kg solvent, and k_f is a constant specific to the solvent.

Which of the following solutions will have the lowest melting point: 2 m sugar (sucrose) or 0.8 m calcium chloride?

First need to look at concentration of particles.

sugar is covalent, so 2 m particles,

calcium chloride is ionic with 1 mole calcium ions and 3 moles of chloride ions for each mole of CaCl₂ = 2.4 m particles,

Therefore the 0.8 m calcium chloride solution will have the higher bp.

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© R A Paselk

Last modified 15 April 2011