16.2. Solution Concentration http://www.ck12.org
Practice Problems- 125 mL of a 2.55 M solution of Zn(NO 3 ) 2 is diluted with water to make the final volume 197 mL. Calculate
the new molarity. - What volume of a 1.50 M solution of KCl needs to be diluted in order to prepare 2.40 L of a 0.0750 M
solution?
Dilutions can be performed in the laboratory with various tools, depending on the volumes required and the desired
accuracy. The image below (Figure16.9) illustrates the use of two different types of pipettes. The use of a
calibrated pipette instead of a graduated cylinder improves accuracy. In the figure on the right, the student is using a
micropipette, which is designed to quickly and accurately dispense very small volumes. Micropipettes are adjustable
and come in a variety of sizes.
FIGURE 16.9
(A) A graduated glass pipette is used to
perform a dilution. (B) A micropipette is
used to deliver small volumes of a liquid.Molality
A final way to express the concentration of a solution is by its molality. Themolality (m)of a solution is the moles
of solute divided by the kilograms of solvent. A solution that contains 1.0 mol of NaCl dissolved in 1.0 kg of water
is a “one molal” solution of sodium chloride. The symbol for molality is a lower-case m written in italics.
Molality(m) =
moles of solute
kilograms of solvent=
mol
kgMolality differs from molarity only in the denominator. While molarity is based on the liters ofsolution, molality
is based on the kilograms ofsolvent. Concentrations expressed in molality are used when studying properties of
solutions related to vapor pressure and temperature changes, as you will see in the subsequent lesson. Molality is
used because its value does not change with changes in temperature. The volume of a solution, on the other hand, is
slightly dependent upon temperature.
Sample Problem 16.5: Calculating Molality
Determine the molality of a solution prepared by dissolving 28.60 g of glucose (C 6 H 12 O 6 ) in 250. g of water.