Page 35 - Modern Analytical Chemistry

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18 Modern Analytical Chemistry

FW 97.994

(a) EW = = = 32.665 N = n ´ M = 3 ´ 6.0 = 18 N
n 3
FW 97.994
(b) EW = = = 48.997 N = n ´ M = 2 ´ 6.0 = 12 N

n 2
FW 97.994

(c) EW = = = 97.994 N = n ´ M = 1 ´ 6.0 = 6.0 N
n 1
2B.3 Molality

molality Molality is used in thermodynamic calculations where a temperature independent
The number of moles of solute per unit of concentration is needed. Molarity, formality and normality are based on the
kilogram of solvent (m).
volume of solution in which the solute is dissolved. Since density is a temperature de-
pendent property a solution’s volume, and thus its molar, formal and normal concen-
trations, will change as a function of its temperature. By using the solvent’s mass in
place of its volume, the resulting concentration becomes independent of temperature.

2B. 4 Weight, Volume, and Weight-to-Volume Ratios

weight percent Weight percent (% w/w), volume percent (% v/v) and weight-to-volume percent
Grams of solute per 100 g of solution. (% w/v) express concentration as units of solute per 100 units of sample. A solution in which
(% w/w).
a solute has a concentration of 23% w/v contains 23 g of solute per 100 mL of solution.
Parts per million (ppm) and parts per billion (ppb) are mass ratios of grams of
volume percent solute to one million or one billion grams of sample, respectively. For example, a steel
Milliliters of solute per 100 mL of
solution (% v/v). that is 450 ppm in Mn contains 450 mg of Mn for every gram of steel. If we approxi-
mate the density of an aqueous solution as 1.00 g/mL, then solution concentrations can
be expressed in parts per million or parts per billion using the following relationships.
weight-to-volume percent
Grams of solute per 100 mL of solution mg mg
(% w/v). ppm = =
liter mL
m g ng
parts per million ppb = =
Micrograms of solute per gram of liter mL
solution; for aqueous solutions the units
are often expressed as milligrams of For gases a part per million usually is a volume ratio. Thus, a helium concentration
solute per liter of solution (ppm). of 6.3 ppm means that one liter of air contains 6.3 mL of He.

parts per billion 2B. 5 Converting Between Concentration Units
Nanograms of solute per gram of
solution; for aqueous solutions the units The units of concentration most frequently encountered in analytical chemistry are
are often expressed as micrograms of molarity, weight percent, volume percent, weight-to-volume percent, parts per mil-
solute per liter of solution (ppb).
lion, and parts per billion. By recognizing the general definition of concentration
given in equation 2.1, it is easy to convert between concentration units.

EXAMPLE 2.2
A concentrated solution of aqueous ammonia is 28.0% w/w NH 3 and has a
density of 0.899 g/mL. What is the molar concentration of NH 3 in this solution?
SOLUTION
g
.
.
28 0 gNH 3 0 899 solution 1 mole NH 3 1000 mL
14 8 M
´ ´ ´ = .
.
g
100 solution m L solution 17 04 gNH 3 liter