# The Mechanism of Chromatographic Retention - The Association of Methanol with Water > Page 31

The molar concentration, in these circumstances, can be defined as the moles per milliliter before mixing. As there is a volume change on mixing, to be precise, equation (7) should be expressed in moles per milliliter after mixing. At the maximum, however, there is a 3% change in volume on mixing and, thus, (k) will, also, only be changed by approximately 3%. Thus, to simplify the algebra (but, nevertheless, maintain a precision of 3% or better) molar concentrations before mixing are used.

If (a) is the volume fraction of methanol in the original mixture, then the volume fraction of water will be (1-a). Now, the molar volume of a substance is the ratio of the molecular weight to the density, consequently, the molar concentrations of methanol and water will be( ) and ( ) respectively (where (VM) and (VW) are the molar volumes of methanol and water respectively.

Thus, (8)

and (9)

Solving for [W] using equations (7), (8) and (9), (10)

where and In addition, (11)

and (12)

As [M], [W] and [MW] were defined in moles per milliliter then the volume (vi) after mixing will be