Parts, Molarity, Molality, Normality, Moles, Mole fraction, Mill/equivalents and normal solutions, Percent, ppm, ppb, ppt & ppq.

Parts

Pharmacopoeias express some concentrations in terms of the number of 'parts' of solute dissolved in a stated number of 'parts' of solution.

Molarity                               : Moles of solute per liter of solution.

[Example: Molecular weight of NaOH is “40” (Na=23; O=16; H=1). When the unit of this number is expressed as “gram” and when this 40 gm of NaOH is dissolved in 1 liter solution this is called “1 molar NaOH Solution”

Molality                                : Moles of solute per kilogram of solvent.

[Example: Molecular weight of NaOH is “40” (Na=23; O=16; H=1). When the unit of this number is expressed as “gram” and when this 40 gm of NaOH is dissolved in 1 kilogram of solvent this is called “1 molal NaOH Solution”

Normality            : Equivalent weight of solute per liter of solution.

[Example: Molecular weight H₂SO₄ is “98” (and equivalent weight of H₂SO₄ is “46”. When the unit of this number is expressed as “gram” and when this 46 gm of H₂SO₄ is dissolved in 1 liter solution this is called “1 normal H₂SO₄ Solution”

Moles                     : When the molecular weight of a compound expressed as mass and unit is “gram”, then this amount is called as mole of that compound.

Mole fraction

This is often used in theoretical considerations and is defined as the number of moles of solute divided by the total number of moles of solute and solvent, i.e.:

                                                         n₁

mole fraction of solute ( x₁ )    = -----------         

                                                     n₁ + n₂

where n\ and n2 are the numbers of moles of solute and solvent, respectively.

Mill/equivalents and normal solutions

The concentrations of solutes in body fluids and  in solutions used as replacements for those fluids are usually expressed in terms of the number of millimoles (1 millimole = one thousandth of a mole) in a litre of solution. In the case of electrolytes, however, these concentrations may still be expressed in terms of milliequivalents per litre. A milliequivalent (mEq) of an ion is, in fact, one thousandth of the gram equivalent of the ion, which is in turn the ionic weight expressed in grams divided by the valency of the ion. Alternatively,

                 ionic weight in mg

1mEq = ----------------------------

                         valency

Percent:

One part per hundred is generally represented by the percent (%) symbol and denotes one part per 100 parts, one part in 10²

ppm:

One part per moillion (ppm) denotes one part per 1,000,000 parts, one part in 10⁶; One ppm is equivalent to 1 milligram of something per liter of water (mg/l) or 1 milligram of something per kilogram solid (mg/kg) (1/1,000,000)

ppb:

One part per billion (ppb) denotes one part per 1,000,000,000 parts, one part in 10⁹, One ppb represents one microgram of something per liter of water (ug/l), or one microgram of something per kilogram of solid (ug/kg). (1/1,000,000,000)

ppt:

One part pertrillion (ppt) denotes one part per 1,000,000,000,000 parts, one part in 10¹²

(1/1,000,000,000,000).

ppq:

One part per quadrillion (ppq) denotes one part per 1,000,000,000,000,000 parts, one part in 10^{15 (}1/1,000,000,000,000,000).