# Dispersion in Chromatography Columns - Resistance to Mass Transfer Dispersion > Page 27

On average, a
molecule will remain in the mobile phase a time (t_{a}) before it is
adsorbed. During this time, it will be moving at the mean velocity of the
mobile phase (u) and will, thus, move a distance (ut_{a}). Thus, in
moving a distance _{}, the total number of adsorptions will be
_{} and
the total number of steps including the adsorption and desorption steps will be
_{}.
It also follows, from the Random Walk Model that,

_{} where
_{}

It is now necessary to determine the average step length (s) to obtain an expression for (H).

The *step
length* is that *length moved by the molecule relative to that of the zone
center* and, while the molecule has move (vt_{a}) during time (t_{a}),
the zone center has also moved. Now, it was shown in the Plate theory that the
zone velocity is _{}where (k") is the *dynamic *capacity ratio of the solute.

Consequently,
_{}.

and, _{},

*i.e.,* _{}