5 Steps to a 5 AP Chemistry

Molality

Sometimes the varying volumes of a solution’s liquid component(s) due to changes in tem-

perature present a problem. Many times volumes are not additive, but mass is additive. The

chemist then resorts to defining concentration in terms of the molality. Molality (m)is

defined as the moles of solute per kilogram of solvent.

m=moles solute/kilograms solvent

Notice that this equation uses kilograms of solvent, not solution. The other concentra-

tion units use mass or volume of the entire solution. Molal solutions use only the mass of

the solvent. For dilute aqueous solutions, the molarity and the molality will be close to the

same numerical value.

For example, ethylene glycol (C 2 H 6 O 2 ) is used in antifreeze. Determine the molality of

ethylene glycol in a solution prepared by adding 62.1 g of ethylene glycol to 100.0 g of water.

The most common error is to use the total grams in the denominator instead of just the

grams of solvent.

Electrolytes and Nonelectrolytes

An electrolyteis a substance that, when dissolved in a solvent or melted conducts an elec-

trical current. A nonelectrolytedoes not conduct a current when dissolved. The conduc-

tion of the electrical current is usually determined using a light bulb connected to a power

source and two electrodes. The electrodes are placed in the aqueous solution or melt, and

if a conducting medium is present, such as ions, the light bulb will light, indicating the sub-

stance is an electrolyte.

The ions that conduct the electrical current can result from a couple of sources. They

may result from the dissociation of an ionically bonded substance (a salt). If sodium chlo-

ride (NaCl) is dissolved in water, it dissociates into the sodium cation (Na+) and the chlo-

ride anion (Cl−). But certain covalently bonded substances may also produce ions if

dissolved in water, a process called ionization. For example, acids, both inorganic and

organic, will produce ions when dissolved in water. Some acids, such as hydrochloric acid

(HCl), will essentially completely ionize. Others, such as acetic acid (CH 3 COOH), will

only partially ionize. They establish an equilibrium with the ions and the unionized species

(see Chapter 13 for more on chemical equilibrium).

Species such as HCl that completely ionize in water are called strong electrolytes, and

those that only partially ionize are called weak electrolytes. Most soluble salts also fall into

the strong electrolyte category.

Colligative Properties

Some of the properties of solutions depend on the chemical and physical nature of the indi-

vidual solute. The blue color of a copper(II) sulfate solution and the sweetness of a sucrose

HCl(aq)→+H (aq) Cl (aq)+ − 100% ionization

CH COOH ( a q) 3 H (aq) CH COO (aq)+ + 3 − partial ionizatiion

molality

62.1g C H O

100.0 g H O

1000 g

1

262

2

=×

kkg

1mol C H O

62.1g C H O

(^262) mC H O
262

×=10 0. 262

Solutions and Colligative Properties  183

KEY IDEA

STRATEGY