The Foundations of Chemistry

You may be familiar with several examples of the effects we have studied. Sea water

does not freeze on some days when fresh water does, because sea water contains higher

concentrations of solutes, mostly ionic solutes. Spreading soluble salts such as sodium

chloride, NaCl, or calcium chloride, CaCl 2 , on an icy road lowers the freezing point of

the ice, causing the ice to melt.

A familiar application is the addition of “permanent” antifreeze, mostly ethylene glycol,

HOCH 2 CH 2 OH, to the water in an automobile radiator. Because the boiling point of the

solution is elevated, addition of a solute as a winter antifreeze also helps protect against

loss of the coolant by summer “boil-over.” The amounts by which the freezing and boiling

points change depend on the concentration of the ethylene glycol solution. The addition

of too much ethylene glycol is counterproductive, however. The freezing point of pure

ethylene glycol is about 12°C. A solution that is mostly ethylene glycol would have a

somewhat lower freezing point due to the presence of water as a solute. Suppose you graph

the freezing point depression of water below 0°C as ethylene glycol is added, and also

graph the freezing point depression of ethylene glycol below 12°C as water is added.

These two curves would intersect at some temperature, indicating the limit of lowering

that can occur. (At these high concentrations, the solutions do not behave ideally, so the

temperatures could not be accurately predicted by the equations we have introduced in

this chapter, but the main ideas still apply.) Most antifreeze labels recommend a 50
 50

mixture by volume (fp 34°F, bp 265°F with a 15-psi pressure cap on the radiator), and

cite the limit of possible protection with a 70
30 mixture by volume of antifreeze
water

(fp 84°F, bp 276°F with a 15-psi pressure cap).

DETERMINATION OF MOLECULAR WEIGHT BY

FREEZING POINT DEPRESSION OR

BOILING POINT ELEVATION

The colligative properties of freezing point depression and, to a lesser extent, boiling point

elevation are useful in the determination of molecular weights of solutes. The solutes must

be nonvolatile in the temperature range of the investigation if boiling point elevations are

to be determined. We will restrict our discussion of determination of molecular weight

to nonelectrolytes.

EXAMPLE 14-10 Molecular Weight from a Colligative Property

A 1.20-gram sample of an unknown covalent compound is dissolved in 50.0 grams of benzene.

The solution freezes at 4.92°C. Calculate the molecular weight of the compound.

Plan

To calculate the molecular weight of the unknown compound, we find the number of moles

that is represented by the 1.20 grams of unknown compound. We first use the freezing point

data to find the molality of the solution. The molality relates the number of moles of solute

and the mass of solvent (known), so this allows us to calculate the number of moles of unknown.

14-13

The total concentration of all dissolved
solute species determines the
colligative properties. As we will
emphasize in Section 14-14, we must
take into account the extent of ion
formation in solutions of ionic solutes.

566 CHAPTER 14: Solutions

Ethylene glycol, HOCH 2 CH 2 OH, is the major component of “permanent” antifreeze. It

depresses the freezing point of water in an automobile radiator and also raises its boiling

point. The solution remains in the liquid phase over a wider temperature range than does

pure water. This protects against both freezing and boil-over.