linked - Investigating the Terror

the chemists specializing in the examination of explosives residue were
Whitehurst and Burmeister, who were assigned to the MAU. One problem
for the prosecution in the World Trade Center case was that the MAU
chemists did not find any residue identifying the explosive. Thus, the
normal way of scientifically determining the main charge was
unavailable. Williams' purported identification of the explosive filled that
void.
a. The Science
1. Defendants
' Capacity to Make 1200 Pounds of Urea Nitrate
Williams calculated the amount of urea nitrate the defendants could have
produced from the amounts of urea and nitric acid that were missing-i.e.,
from the amounts ordered minus the amounts recovered in searches
of premises associated with the defendants. To make such a calculation,
the area of chemistry known as stoichiometry must be applied.
Stoichiometry concerns molecular weight relationships in chemical
reactions. In this instance, the chemical reaction was: one molecule of
urea plus one molecule of nitric acid produces one molecule of urea
nitrate. As previously noted, each of these molecules has a different mass
or weight. The molecular weight of urea is 60; that of nitric acid is 63; and
that of urea nitrate is 123. Thus theoretically (100% yield), 60 grams of
urea plus 63 grams of nitric acid produces 123 grams of urea nitrate. For
every 60 grams of urea, 63 grams of nitric acid is required. (Similarly, for
every 60 pounds of urea, 63 pounds of nitric acid is needed.)
Determining the potential amount of urea nitrate that could have been
produced requires a determination, first, of the limiting reagent because
it is the chemical that will run out first. For example, with only 63 grams
of nitric acid, one could only produce 123 grams of urea nitrate even with
an unlimited amount of urea. In this example, the nitric acid would be the
limiting reagent.
Once the limiting reagent is determined, the potential amount of urea
nitrate can be determined with a simple calculation: If urea was the
limiting reagent, for every 60 grams (60 pounds) of urea that was
missing, the perpetrators potentially could have produced 123 grams