Soil Chemistry Ch. 2 Chemical Principles As Applied to Soils I. Chemical units a. Moles and Avogadro’s number The numbers of atoms, ions or molecules are important in chemical reactions because the number, rather than mass or volume, will determine the extent of a reaction. The basic unit is MOLE. 1 mole = Avogadro’s number (6.023 x 10 23 ) entities. Entities can be carbon, iron or water. b. Mass to mole conversion The conversion factor from mole to mass is the gram-atomic or gram-molecular unit, which is the mass in grams of 1 mole of atoms, ions or molecules. For chemical purposes, ions have the same mass as their atoms, because the mass of electrons are insignificant. The molecular weight is the sum of the atomic weights of the ions or atoms in the molecule. For example: 1 mole of C has a mass of 12 grams and contains 6.023x10 23 carbon atoms. 1 mole of O 2 gas has a mass of 32 grams and contains 6.023x10 23 O 2 molecules or 2 x (6.023 x 10 23 ) = 12.06 x 10 23 O atoms. 1 mol of H 2 O has a mass of 18 grams and contains 6.023 x 10 23 H 2 O molecules. c. Concentrations in liquid solutions Gravimetry or the measuring of the mass of products and reactants is the most accurate form of chemical measurement. However, most chemical reactions are carried out in solutions that are measured more conveniently by volume. In fact, there are several ways of expressing the concentration of a substance in solution. 1.) in qualitative terms , e.g. dilute, concentrated, saturated, undersaturated, or oversaturated. 2.) Mass of solute in mass of solvent Ex. 10 mg of NaCl in 1 kg of water or ppm, which stands for part per million. Ex. 25 ppm Na in solution is equivalent to 25 mg Na in 1 kg water. because 25 mg/1 kg = 25 mg/1,000,000 mg = 25 ppm TPSS 435
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Soil Chemistry Ch. 2 Chemical Principles As Applied to Soils
I. Chemical units
a. Moles and Avogadro’s number
The numbers of atoms, ions or molecules are important in chemical reactions because
the number, rather than mass or volume, will determine the extent of a reaction. The
basic unit is MOLE. 1 mole = Avogadro’s number (6.023 x 1023) entities. Entities can
be carbon, iron or water.
b. Mass to mole conversion
The conversion factor from mole to mass is the gram-atomic or gram-molecular unit,
which is the mass in grams of 1 mole of atoms, ions or molecules. For chemical
purposes, ions have the same mass as their atoms, because the mass of electrons are
insignificant. The molecular weight is the sum of the atomic weights of the ions or
atoms in the molecule.
For example: 1 mole of C has a mass of 12 grams and contains 6.023x1023 carbon
atoms.
1 mole of O2 gas has a mass of 32 grams and contains 6.023x1023 O2 molecules or 2
x (6.023 x 1023) = 12.06 x 1023 O atoms.
1 mol of H2O has a mass of 18 grams and contains 6.023 x 1023 H2O molecules.
c. Concentrations in liquid solutions
Gravimetry or the measuring of the mass of products and reactants is the most accurate
form of chemical measurement. However, most chemical reactions are carried out in
solutions that are measured more conveniently by volume. In fact, there are several
ways of expressing the concentration of a substance in solution.
1.) in qualitative terms,
e.g. dilute, concentrated, saturated, undersaturated, or oversaturated.
2.) Mass of solute in mass of solvent
Ex. 10 mg of NaCl in 1 kg of water or ppm, which stands for part per million.
Ex. 25 ppm Na in solution is equivalent to 25 mg Na in 1 kg water.
because 25 mg/1 kg = 25 mg/1,000,000 mg = 25 ppm
TPSS 435
Soil Chemistry Ch. 2 3.) Molarity: (M)
The number of moles of solute per liter of solution.
Molarity is often used for dilute solutions. EX: 1 molar solution of CaCl2 is
prepared by dissolve 1 mole (or 147g) of CaCl2 * 2 H20 in water and making the
solution up to 1 liter.
4.) Molality
moles per 1000 g of solvent. Since mass, unlike volume, is temperature
independent; molality is a more exact expression and often used for concentrated
solutions or solutions at high temperatures.
5.) Mole fraction
mole of solute / total moles of (solute + solvent)
Ex. 1mole of NaCl in 1L of water has a mole fraction of…
# moles of water in 1 liter = 1000 g/ 18 g = 55.5
Thus, 1/(1+55.5) = 1/56.5
You should be able to convert... ppm molarity or vice versa
Examples: 15 ppm Ca = 15 mg Ca/1 L = 15/40 mmole ca/1 L = 0.375 mM
6.) Normality (N) equivalent per liter of solution. Equivalents are the number of
moles divided by either:
a. the number of moles of H+ or OH- that react with 1 mole of the substance.
EX: Ca(OH)2 + 2H+ ==> Ca 2+ + 2H2O
1 M Ca(OH)2 = 2 N Ca(OH)2
or
b. the number of “moles” or Faradays of electrons that one mole of the substance
accepts or donates. This definition is more applicable to an oxidation-reduction
(redox) reaction.
TPSS 435
Soil Chemistry Ch. 2 Ex. Oxidation reaction of HClO4 in the wet digestion method of plant tissue
analysis.
HClO4 + 8 e- + 7H+ <= Cl- + 4H2O
1 M HClO4 = 8 N HClO4
7.) The “p” scale
Low concentrations are sometimes expressed as negative logarithm of molarity
or the ‘p’ scale.
Ex. pH which is –log(H+) Or pH2PO4, pCa which are –log(H2PO4) and –log (Ca 2+ ), respectively.
In the ‘p’ scale the smaller the concentrations or activities the larger the number.
Ex. [H+] = 10-4 M pH = 4
[H+] = 10-6 M pH = 6
d. Concentrations in gaseous solutions at low pressure (1 atm or less)
At low pressure, gases behave almost ideally, and the volume of a gas is independent of
other gases present. The volume of all gases at 0 0C and 1 atm is 22.4 L/mole
Concentrations of a gas are often expressed as:
1.) partial pressure - which is the ratio of pressure of a gas to total pressure of all
gases in a mixture, or
2.) mole fraction – number of moles of a gas / total moles of all gases in a mixture
at low temp. and gas behaves ideally. partial pressure = (identical to) mole
fraction
3.) ppm – partial pressure x 106
ex. CO2 concentration in the atmosphere has currently a partial pressure of
0.000381 or 0.000381 x 106 ppm = 381 ppm.
4.) % - partial pressure x 102
for the above example CO2 = 0.000381 = 0.0381%
TPSS 435
Soil Chemistry Ch. 2
II. S.I. Units – S.I. stands for Le Systeme Internationale d’unites, which is a slight
modification of the metric units, and has been adapted as the official units in soil science
publications.
a. Basic Units – there are 7 fundamental units (Table 1.6- pg. 22) which are meter
(length), kilogram (mass), second (time), ampere (electric current), Kelvin (thermo
temp), mole (amount of substance), and light intensity. Several other units can be
derived from these basic units as you can see in the same Table (Table 1.6- pg. 22),
such as force, pressure and electrical conductivity. Prefixes should be used for very
large or small numbers as you can see in the table 1.7 on page 23
One of the units that is often used in soil chemistry is meq/100g for cation exchange
capacity expression.
In the S.I. system meq/100g = 1 cmol(+)kg-1 = 10 mmolc kg-1