Page 47 - Modern Analytical Chemistry

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30 Modern Analytical Chemistry

2E Preparing Solutions

Preparing a solution of known concentration is perhaps the most common activity
in any analytical lab. The method for measuring out the solute and solvent depend
on the desired concentration units, and how exact the solution’s concentration
needs to be known. Pipets and volumetric flasks are used when a solution’s concen-
tration must be exact; graduated cylinders, beakers, and reagent bottles suffice when
concentrations need only be approximate. Two methods for preparing solutions are
described in this section.

2E.1 Preparing Stock Solutions

stock solution A stock solution is prepared by weighing out an appropriate portion of a pure solid
A solution of known concentration from or by measuring out an appropriate volume of a pure liquid and diluting to a
which other solutions are prepared.
known volume. Exactly how this is done depends on the required concentration
units. For example, to prepare a solution with a desired molarity you would weigh
out an appropriate mass of the reagent, dissolve it in a portion of solvent, and bring
to the desired volume. To prepare a solution where the solute’s concentration is
given as a volume percent, you would measure out an appropriate volume of solute
and add sufficient solvent to obtain the desired total volume.

EXAMPLE 2. 9

Describe how you would prepare the following three solutions: (a) 500 mL of
approximately 0.20 M NaOH using solid NaOH; (b) 1 L of 150.0 ppm Cu 2+
using Cu metal; and (c) 2 L of 4% v/v acetic acid using concentrated glacial
acetic acid.

SOLUTION
(a) Since the concentration only needs to be known to two significant figures,
the mass of NaOH and volume of solution do not need to be measured
exactly. The desired mass of NaOH is
.
020 mol 400 g
.
.
´ ´ 050 L = 4.0 g
L mol
To prepare the solution we place 4.0 g of NaOH, weighed to the nearest tenth
of a gram, in a bottle or beaker and add approximately 500 mL of water.
2+
(b) Since the concentration of Cu needs to be exact, the mass of Cu metal
and the final solution volume must be measured exactly. The desired mass
of Cu metal is
150.0 mg
.
´ 1 000 L = 150.0 mg = 0.1500 g
L
To prepare the solution we measure out exactly 0.1500 g of Cu into a small
beaker. To dissolve the Cu we add a small portion of concentrated HNO 3
and gently heat until it completely dissolves. The resulting solution is
poured into a 1-L volumetric flask. The beaker is rinsed repeatedly with
small portions of water, which are added to the volumetric flask. This
quantitative transfer 2+
The process of moving a sample from process, which is called a quantitative transfer, ensures that the Cu is
one container to another in a manner completely transferred to the volumetric flask. Finally, additional water is
that ensures all material is transferred. added to the volumetric flask’s calibration mark.