Chapter 11 1

3. Units of Concentration

molestotal

Amolesunitless

%100masstotal

Amass

Definition

same

same

Chapter 11 2

Advantages/disadvantages of each (read only; no lecture)

Chapter 11 3

Example problems1. Determine the molarity of a 0.258 m solution of glucose given that the solution’s density is 1.0173 g ml-1 and that the molar mass of glucose is 180.2 g.

•first, write down definitions: molarity is moles solute per liter solution; molality is moles solute per kg solvent; density is mass solution per volume solution

•assume some amount of solution to start: in this case 0.258 moles glucose and 1 kg water

we want: moles solute/liter solution

so we have 0.258 moles solute

Chapter 11 4

•get the total volume of the solution - glucose plus water

•solve for the molarity

nsol'1046.49water1000glucose46.49

glucose49.462.180

258.0

ggg

gmol

gmoles

mlg

mlnsolg 70.1028

0173.1

1'49.1046

mass sol’n

volume sol’n

ML

moles251.0

02870.1

258.0

Chapter 11 5

2. Determine the molality of a 0.500 M solution of acetic acid (molar mass 60.02 g) with a density of 1.0042 g ml-1.

•Again, write down the units and their definitions.

•Start by assuming 1 L of solution and thus 0.500 moles acetic acid

•determine the mass of the solution from the density times the volume

•determine the mass of water (in kg) by subtraction (subtract the mass of the acetic acid from the mass of the solution)

•solve for the molality

Chapter 11 6

3. Describe how 1.50 L of a 12.0% KBr solution is made if the solution density is 1.10 g ml-1. In other words, determine the mass of KBr and the mass of water to be mixed together.

•Write out the units and their definitions

•1.50 L or 1500 ml of solution are required; determine the total mass of solution required (HOW?).

•Of the total mass of solution, how much must be KBr?

•Determine the mass of water necessary (HOW?).

Chapter 11 7

gmlgml 165010.11500

KBr198165012.0 g

water1452KBr198nsol'1650 ggg

total mass solution

mass KBr

mass water

dissolve 198 g KBr in 1452 g water