Geoscience Reference
In-Depth Information
M
(e.g., a 0.50-
M
solution). Recognize that molarity is moles of solute per liter of solution,
not per liter of solvent. Also recognize that molarity changes slightly with temperature
because the volume of a solution changes with temperature.
5.
Molality
. Molality is used for calculations of colligative properties; it is the number of
moles of solute dissolved in 1 kilogram of solvent. Notice the two key differences between
molarity and molality. Molality uses mass rather than volume and uses solvent instead of
solution:
Molesofsolute
Kilogramsofsoluti
Molality
=
(3.2)
on
Unlike molarity, molality is independent of temperature because mass does not change
with temperature. If we were to place 90 g of glucose (0.50 mol) in a flask and then add
1 kg of water we would have a 0.50-molal solution. Molality is usually denoted with a
small
m
(e.g., a 0.50-
m
solution).
6.
Parts per million
. Parts per million (ppm) works like percent by mass but is more conve-
nient when there is only a small amount of solute present. Parts per million is defined as
the mass of the component in solution divided by the total mass of the solution multiplied
by 10
6
(one million):
Mass of component
Mass of
Partsper million
=
×1 000 000
,
,
(3.3)
solution
A solution with a concentration of 1 ppm has 1 gram of substance for every million grams of
solution. Because the density of water is 1 g/mL and we are adding such a tiny amount of sol-
ute, the density of a solution at such a low concentration is approximately 1 g/mL. Therefore,
in general, 1 ppm implies 1 mg of solute per liter of solution. Finally, recognize that 1% =
10,000 ppm. Therefore, something that has a concentration of 300 ppm could also be said to
have a concentration of (300 ppm)/(10,000 ppm/percent) = 0.03% percent by mass.
7.
Parts per billion
. Parts per billion (ppb) works like above, but we multiply by 1 billion (10
9
)
(be aware that the word “billion” has different meanings in different countries). A solution
with 1 ppb of solute has 1 µg (10
-6
) of material per liter.
8.
Parts per trillion
. Parts per trillion (ppt) works like parts per million and parts per billion
except that we multiply by 1 trillion (10
12
). There are few, if any, solutes that are harmful at
concentrations as low as 1 ppt.
The following examples can help in formalizing these different forms. In these examples, sub-
scripts for components are
i
= 1, 2, 3, …,
n
, and subscripts for phases are
g
= gas,
a
= air,
l
= liquid,
w
= water, and
s
= solids and soil.
3.4.1 m
aterial
C
ontent
: l
liquid
p
hases
Mass concentration, molar concentration, or mole fraction can be used to quantify material content
in liquid phases:
i
,
=
Mass of material
Volume of water
i
Mass concentrationofcomponent
i
in wateri
=
p
(3.4)
Molesofmaterial
Volume of water
i
Molarconcentrationofcomponent
i
in wateri
===
C
(3.5)
iw
,
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