The Foundations of Chemistry

Molality

The molality,m,of a solute in solution is the number of moles of solute per kilo-

gram of solvent.

molality

EXAMPLE 14-1 Molality

What is the molality of a solution that contains 128 g of CH 3 OH in 108 g of water?

Plan

We convert the amount of solute (CH 3 OH) to moles, express the amount of solvent (water)
in kilograms, and apply the definition of molality.

Solution



 37.0 mCH 3 OH

You should now work Exercise 28.

EXAMPLE 14-2 Molality

How many grams of H 2 O must be used to dissolve 50.0 grams of sucrose to prepare a 1.25 m
solution of sucrose, C 12 H 22 O 11?

Plan

We convert the amount of solute (C 12 H 22 O 11 ) to moles, solve the molality expression for kilo-
grams of solvent (water), and then express the result in grams.

Solution

_?_ mol C 12 H 22 O 11 50.0 g C 12 H 22 O 11 0.146 mol C 12 H 22 O 11

molality of solution

Rearranging gives

kg H 2 O

0.117 kg H 2 O 117 g H 2 O

0.146 mol C 12 H 22 O 11



1.25 mol C 12 H 22 O 11 /kg H 2 O

mol C 12 H 22 O 11



molality of solution

mol C 12 H 22 O 11



kg H 2 O

1 mol C 12 H 22 O 11



342 g C 12 H 22 O 11

37.0 mol CH 3 OH



kg H 2 O

1 mol CH 3 OH



32.0 g CH 3 OH

128 g CH 3 OH



0.108 kg H 2 O

_?_ mol CH 3 OH



kg H 2 O

g CH 3 OH mol CH 3 OH

g H 2 O kg H 2 O

molality

number of moles solute



number of kilograms solvent

Molality is based on the amount of

solvent not solution.

14-8 Molality and Mole Fraction 555

In other examples later in this chapter

we will calculate several properties of

the solution in Example 14-2.

Each beaker holds the amount of a

crystalline ionic compound that will

dissolve in 100. grams of water at

100°C. The compounds are (top row,

left to right)39 grams of sodium

chloride (NaCl, white), 102 grams of

potassium dichromate (K 2 Cr 2 O 7 ,

red-orange), 341 grams of nickel

sulfate hexahydrate (NiSO 4 6H 2 O,

green); (bottom row, left to right)

79 grams of potassium chromate

(K 2 CrO 4 , yellow), 191 grams of

cobalt(II) chloride hexahydrate

(CoCl 2 6H 2 O, dark red), and

203 grams of copper sulfate

pentahydrate (CuSO 4 5H 2 O, blue).