2.3 Partial pressure units
Atmospheric pressure is the force per unit area exerted on the
Earth’s surface by the weight of the atmosphere above it. A column
of air extending from the Earth’s surface to the top of the atmosphere
has a mass of ~1 kilogram and, by definition, exerts a pressure of 1
atm.
~1% trace gases
20.9% O2
That column of air consists of a mixture of gases each representing a
volume fraction, e.g. 78.1% of that column of air is nitrogen - a
volume fraction of 0.781.
The partial pressure of nitrogen is equal to the product of that volume
fraction and the total atmospheric pressure,
Pi  volume fraction  Ptotal
For a total atmospheric pressure (Pi) of 1 atm, the partial pressure
of nitrogen in air is,
Pi  0.7811 atm  0.781 atm
78.1% N2
2.5 Other types of units
Equivalent weight
The number of equivalents per mole that a chemical species represents is, for acids, the number of moles of
protons that it can donate; and, for bases, the number of moles of H+ that would react with that base.
Acids
HCl  1H   Cl 
1 equivalent
H 2 SO4  2 H   SO42

Bases
3
4
H 3 PO4  3H  PO
2 equivalents
3 equivalents
NaOH  Na   OH 
2
2
3
CaCO3  Ca  CO
3
4
PO
1 equivalent
2 equivalents
3 equivalents
The equivalent weight of a substance is defined as its molecular weight divided by the number of equivalents
contributed per mole. For CaCO3,
Molecular weight of CaCO3  40  12  3 16  100
Equivalents contributed by CaCO3  2
Equivalent weight of CaCO3  50
Thus the gram equivalent weight (gew) of CaCO3 is 50, i.e. there are 50g of CaCO3 per equivalent.
Concentration as a common constituent
As rainwater passes over bedrock containing limestone and dolomite, calcium
(Ca) and magnesium (Mg) carbonates are dissolved and enter the water as
calcium (Ca2+) and magnesium (Mg2+) ions. Once in our water supply
systems, these ions may precipitate forming bathtub scum and a hard scale
which clogs pipes and reduces the efficiency of hot water heaters. The
“hardness” of a water is equal to the sum of its divalent ions (e.g. Ca2+, Mg2+,
Fe2+, Mn2+, Sr2+).
It is cumbersome to seek expression of the five hardness constituents.
Instead, these are expressed as a common constituent, traditionally CaCO3,
and summed to yield the total hardness. Expression of the constituents as
CaCO3 is accomplished by multiplying the concentration of the constituent by
the ratio of the gew for CaCO3 to the gew for the constituent,
CaCO3 ; MW  100, eq  2, gew  50
Ca 2 ; MW  40, eq  2, gew  20
Mg 2 ; MW  24, eq  2, gew  12
mgCa 2 50
mg

 125
as CaCO3
L
20
L
mgMg 2 50
mg
25
  104
as CaCO3
L
12
L
50
Total Hardness  229
mg
as CaCO3
L
This is very hard water and would merit
treatment for use as a domestic supply.
Particle concentrations
The total solids content (TS, mg/l) of a water sample may be divided into
dissolved (e.g. salts) and suspended (e.g. particulates) fractions. Each of
these may be further divided into inorganic and organic fractions.
Total suspended solids (TSS, mg/L) represents the concentration, after drying,
of the particles retained on a filter. The filter is then ignited and the material
remaining (ash) is termed fixed suspend solids (FSS, mg/L) and that lost
volatile suspended solids (VSS, mg/L). FSS and VSS represent the inorganic
and organic fractions, respectively. Total dissolved solids (TDS, mg/L) are
those which pass through a filter and are quantified by drying the sample.
http://www.hhpsd.com/whardness.htm
TS
VSS
FSS
TSS
Several of these fractions have significance in
environmental engineering applications:
• TDS (saltiness) is important with respect to
drinking water and irrigation;
• TSS (as turbidity) is used as a standard for
safe drinking water consumption; and
TDS
• VSS (organic matter) provides a measure of
a water to consume oxygen.