Zinc is often weighed as ZnNH4P04 : in this case for
each molecule of the precipitate there will be but one
zinc and the proportion will be

178.47 : 65.4 : : wt. ppt. : x.
All direct stoichiometrical calculations are made in
this way, using the proper atomic weights and bear-
ing in mind to place in the proportion the weight of the
number of atoms or molecules of the constituent sought,
which the precipitate or the residue weighed contains.
Indirect Determinations. — In the material weighed
the element to be determined may not be present at all.
For example : Potassium is precipitated quantita-
tively as K2PtCl6 : this may be weighed as such or may
be ignited with a little oxalic acid, the soluble potassium
chloride washed out, and the spongy platinum weighed.
The reactions are

2KCI + HgPtCle = K2PtCl6+ 2HCI.
To calculate the weight of KgO from the weight of the
double chloride we have

Mol. wt. KsPtCle : Mol. wt. KgO : : wt. ppt. : x.
485.8 : 94.3 : : wt. ppt. : x.
If platinum is weighed : — each molecule of KgPtCle
has one atom of platinum and two of potassium, so for



40 QUANTITY OF REAGENTS.
each atom of platinum weighed there must have been
two atoms of potassium, equivalent to a molecule of
K2O in the solution to cause its precipitation, hence we
have

At, wt. Pt : Mol. wt. KgO : : wt. residue : x
or 194.8 : 94.3 : : wt. residue : x

and in either case -. — X 100 = Percent of K3O.
