Sanitary Engineering Principles - Mercer University



Sanitary Engineering Principles

I. Chemistry Fundamentals

1) Atomic Weight: weight of an atom of an element relative to the atomic mass unit (a.m.u.) of Carbon (C), which is 12.0. a.m.u.

2) Gram Molecular Weight: molecular weight of a compound expressed in grams. (GMW)

Example: 1 mole of CaCO3 = 40 = 12 = 3x16 = 100 gms.

Therefore, the gram molecular weight of CaCO3 equals 100 grams.

(3) Molarity (M): moles of solute, expressed as moles/liter.

Vol. of solution in liters

= (weight of solute, grams)/GMW

vol. of solution in liters

(4) Molality: moles of solute, expressed as moles/kg.

1,000 grams of solution.

IMPORTANT POINT: A 1 Molar solution is prepared by diluting 1

mole of a substance with distilled H2O to a final volume of 1 liter. A 1

Molal solution is prepared by placing 1 mole of a substance into 1 liter of

distilled H2O.

(5) Normality (N): equivalents of solute, expressed as eq/liter.

Vol. of solution in liters

= (weight of solute, grams) /GEW

vol. of solution in liters

6) Gram Equivalent Weight (GEW): GEW = GMW

Valence or number of H+ ions displaced.

IMPORTANT POINT: N = v(M), where v is equal to the valence or

number of H+ ions displaced.

(7) Equivalence: e = N(V)

where e = number of equivalent of a substance present

N = Normality, (eq/1)

V = Volume of solution, (liters).

IMPORTANT POINT: Equivalents react with equivalents. Important point

when considering acid-base or oxidation-reduction

reactions.

Each equivalent corresponds to one mole of H+ or one mole of electrons. Since the product of normality times volume yields equivalents, the simple relationship exists for a stoichiometric reaction.

N1V1 = N2 V2

Where N2, N2 = the normality of two solutions

V1, V2 = the volume of each solution.

|Reactant Product |GMW, gr/mole |GEW, gr//eq |

|[pic] |36.5 |36.5 |

|[pic] |98 |98 |

|[pic] |98 |49 |

|[pic] |98 |98 |

|[pic] |98 |49 |

|[pic] |98 |32.7 |

|[pic] |58.5 |58.5 |

|[pic] |106 |53 |

|[pic] |100 |100 |

|[pic] |100 |50 |

|[pic] |107 |35.7 |

Milliequivalents per Liter.

Expression of concentrations in terms of milliequivalents per liter is useful in determining ionic balances based on the complete mineral analysis of a water. When milliequivalents of all cations present (positively charged ions) are added together and compared with the milliequivalents of the anions (negatively charged ions), they should be equal owing to the electrical neutrality of aqueous solutions. This is called the “electroneutrality condition”. If the analysis of a water does not exhibit a close balance between cations and anions, either the analysis is in error or an important ionic constituent has been neglected.

Millimoles per Liter

The mole is the most fundamental unit in chemistry. It represents a number, just as a “dozen” represents a number. In the case of the mole, the number is Avagadro’s number, 6.02 x 1023. Thus, the expression of the molar concentration of an ionic constituent tells you the exact number of ions, atom, or molecules of that constituent in a liter of solution.

For most natural waters, concentrations may be expressed in millimoles per liter (10-3 M), for convenience, since the concentration of major constituents are in that order of magnitude. Minor constituents of natural water, such as iron, manganese, fluoride, phosphate and ammonium ion, may range down to 10-5 M.

EXAMPLE: Convert the concentrations given from millieequivalents per liter to millimoles per liter.

|Solution |meq/L |mM/L |

|Ca++ |2.5 |1.25 |

|Mg++ |2.1 |1.05 |

|Na+ |0.5 |0.5 |

|[pic] |3.2 |3.2 |

|[pic] |1.1 |0.55 |

|[pic] |0.7 |0.7 |

Example Calculation: [pic]

Miligrams per Liter

The most commonly used unit for the expression of concentration of aqueous constituents is milligrams per liter. This is obtained by multiplying the molar concentration by the molecular weight.

EXAMPLE: Convert the concentrations given above in millimoles per liter to milligrams per liter.

| |mM/1 | |Molecular | |Mg/1 |

| | | |Weight, mg/mM | | |

|Ca++ |1.25 |X |40 |= |50 |

|Mg++ |1.05 |X |24 |= |25 |

|Na+ |0.5 |X |23 |= |12.5 |

|[pic] |3.2 |X |61 |= |196 |

|[pic] |0.55 |X |96 |= |53 |

|[pic] |.7 |X |35.5 |= |25 |

2. Oxidation numbers (valence numbers, oxidation states). A number assigned to an atom to follow electron shifts in reactions.

Atoms have oxidation numbers of zero (02, C12, 12, Zn, Fe)

Group I elements (Li, Na, K, Rb, Cs, Fr): +1

Group II elements (Be, Mg, Ca, Sr, Va, Ra): +2

Hydrogen: +1 (except in hydrides: -1) – SELDOM-1

Oxygen: -2 (except in peroxides: -1 – SELDOM –1

For neutral chemical speices, the summation of oxidation numbers equals zero.

+2 +4-6 = 0 +1 +7 –8 = 0 +6 –6 = ) -4 + 4 = 0 =4 –4 = 0

Ca C 03 K Mn 04 Fe203 C H4 C 02

Chemical equations can be balanced by use of oxidation numbers.

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