Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2015

Lecture Notes 35: 24 April

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 Solubility, cont.

Liquid solutions

"Like dissolves like."

 Concentration Measures

Concentration Terms

Percent Concentration

Molarity = M = 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. (see Lecture 13, 20 February)

    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 13, 20 February).

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

Mole fraction = X = moles of solute dissolved in total moles of solution, X = na /Sigman

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:

P = 0.79 (23.76 mmHg) = 18.77 mmHg = 19 mmHg

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© R A Paselk

Last modified 24 April 2015