2HCl + Zn = ZnCl2 + H2. Here the H of the acid is replaced by the Zn.
KCl + AgNO3 = AgCl + KNO3. Here the Ag and the K change places.
(7) The Chemical Arithmetic by which from the molecular weights of two substances, and the weight of one substance we are enabled to get the weight of the required substance is called Stoichiometry.
Example: Required the amount of zinc necessary to generate 10 grams of hydrogen.
Atomic weights of H, Cl, and Zn are respectively 1, 35.5, and 65.3.
The reaction is as follows:—
Zn + 2HCl = ZnCl2 + H2, and shows that 2 atoms of H are used for every 1 of Zn.
| (Mol. Wt. Zn.) | (Mol. Wt. H2) | (Wt. Zn.) | (Wt. H2.) | |||
| 65.3 | : | 2 | = | x | : | 10 |
65.3 × 102 = x = 326.5 grams of Zn.
(8) Berthollet’s Law.—Berthollet established the following law, which is of great importance. When two substances can form a substance insoluble or volatile, under the condition of the reaction, that substance will be formed till one of the factors is exhausted.